First, there is a HUGE range of global issues to care about:

global human rights issue, such as the fight against female genital mutilation, or the fight for education for girls and why the education and prosperity of girls and women benefits and entire community or country, or child labor, or child marriage, or why there needs to be prison reform world wide, or the global slave trade, etc. global climate change's effects on people living in low-lying areas, who are already being flooded out of homes and farms and ranches emergency medical care providing longer-term, sustainable health care health care to address a specific disease or condition (HIV/AIDS, tuberculosis, malaria, Fistula) disappearing wildlife

pollution (industrial waste, strip-mining, nuclear waste, etc.) disaster relief micro-finance for micro-businesses adult literacy universal K-12 education university education access people with disabilities computer access and literacy religious freedom / anti-blasphemy laws children orphans internally-displaced people refugees hunger nutrition sustainable farming

There are programs helping to get bicycles to people in the developing world, and helping educate local people on repairing such, programs to help women regarding computer and Internet literacy, programs to help people in developing countries understand how to address street/stray dog issues, initiatives to use theater/performance as a tool for education and development... it's impossible to list every possible international cause you could support.

There might be an organization in your area that is focused on helping a specific international cause, or is helping a specific country. A good way to find out if this is happening in your area is to look through the volunteering opportunities that are posted to all the major volunteer matching web sites.

VolunteerMatch

Idealist/Action Without Borders

All for Good

Do Something

WomenOnCall.Org

If you are in the UK (England, Scotland, Wales, Ireland), you can join a VSO supporter group, and provide support for currently-serving VSO volunteers and their families, help promote the work of VSO locally in your community, and raise awareness in your community of issues affecting people in developing countries.

If you have skills to offer (translation, web design, database design, business advise, etc.), you can volunteer online to support a variety of organizations working in developing countries. The United Nations Online Volunteering Service is a great place to start. Nabuur, which recruits online volunteers to support organizations working in or for the developing world, and NetSquared, a place where nonprofits, corporations, government agencies, NGOs and individuals propose ideas that involve "the intersection of technology and social impact," are two other great sites to find international causes you can volunteer for online, IF you have skills to offer.

Join the United Nations online community, the Messengers of Humanity, and use your social media voice to advocate for the world’s most vulnerable people.

If you cannot find a volunteering opportunity through any of the aforementioned sites that match your skills and interests, don't give up! You still can still help internationally! First, you will need to pick one issue or one country/region to concentrate on, at least at first. Type different phrases into Google.com and read, read, read. Take your time to learn. If you feel overwhelmed, take a deep breath and keep reading. Don't worry about picking the right cause - you just want to pick the cause that you feel strongly about, whatever that cause is. And you can change your mind; you can choose to support one cause for a year, and then switch to a different cause.

And then what?

• Once you pick your issue, educate yourself about that issue. Visit the web sites of organizations that address this issue and read their web sites (don't just scan them). Read newspaper stories about the issue. Subscribe to the email newsletters of the organizations you think are doing the best/most important work, if they have such, and read them whenever they come. If there is a public event that is going to talk about that issue, attend it. Your goal is to become knowledgeable about the global or international issue you care about.

• Never say or imply that you are somehow an official representative of any organization. You are a concerned individual, but you are not a representative of any organization.

• Occasionally use your status update on FaceBook, Twitter and any other online social networking site you use to talk about what these organizations are doing. It can be this simple:

  CARE International is opening a new school in Afghanistan. Have a look at the web site to learn more!

  Kiva.org has just posted new profiles for women in Peru who want to start small businesses. One wants to start a motorcycle taxi service!

  PeaceCorps has a volunteer in Kenya trying to raise money for a girls school. Go to this web site for complete details!

• Create a display/presentation or an online video and web site that helps students at area high schools and junior high schools, Girl Scouts in your area, or the community in general to understand the international issue. Don't just do statistics; present stories of individual girls or women who have been affected by this issue (you can find profiles on various web sites of NGOs addressing these issues). Put faces to the issue. Emphasize what the girls or students in your area can do to address these issues (writing letters to their congress people and the President expressing their concern, having a fundraiser for a nonprofit that addresses the issue, using their FaceBook status updates to point their network to organizations addressing the issue, staying informed about what's happening, etc.). You could do this presentation for communities of faith, civic groups and professional associations, educating adults about the issues. You could tie your presentation to activities that would help Girl Scouts get a badge by the end of the day. You could also blog about your experience as you research whatever issue you choose, and encourage your friends, family and, as applicable, Girl Scouts in your area, to read it, or use your FaceBook status updates to talk about what you are discovering as you research whatever issue you choose, to further educate your friends, family and others about such.

• Turn your birthday party into a fundraiser for your cause. Invite friends to your house or to a restaurant, and ask in your invitation that, in lieu of gifts, people make donations to a nonprofit organization addressing the cause you support. Let attendees know how you will track donations and how you will get them to the organization, if they want to give cash directly to you. Print off the confirmation of your donation on their behalf and send it to everyone who donated, to confirm you did what you said you would.

• Host a party, cookout or reception at your home, invite your friends (and encourage them to invite their friends), and show a film or documentary relating to the cause you want to educate your friends about. In your invitation, note clearly that this is a fundraiser for a particular organization and that you will be asking for donations; do NOT wait until the party, cookout or reception to tell invitees that you have invited them there in order to ask for donations. Let attendees know how you will track donations and how you will get them to the organization, if they want to give cash directly to you. Print off the confirmation of your donation on their behalf and send it to everyone who donated, to confirm you did what you said you would.

• Have a garage sale to raise money for the organization. Ask friends, relatives, neighbors and others to donate items for your sale. Have large signs at the garage sale that say you are raising money for a particular organization. Make it clear to those who buy items that they will not receive any tax deduction for purchasing any item, nor for making any donations directly to you. Have information about the organization you are supporting that people can easily see and read. Write down the amount of every sale and what the person bought.

• Work a series of jobs in the informal sector (walk dogs, pet sit, provide child care/baby sit, do yard work for neighbors and friends, etc.) and save some or all of the money you make for your cause. Make it clear to those who buy items that they will not receive any tax deduction for employing your services, nor for making any donations directly to you.

• Sell items on eBay. Ask friends, relatives, neighbors and others to donate items for your sale. Take the money that you raise and donate it to the organization.

BPEACE, which supports people in Afghanistan and Rwanda who are trying to start small businesses.

CARE International, which supports women and girls in developing countries.

MADRE.org, which also supports women and girls in developing countries.

Americans for UNFPA, which builds moral, political and financial support within the USA for UNFPA, the United Nations Population Fund, which provides women’s health care and promotes the rights of women in 150 countries. UNFPA is the largest multilateral source of such assistance.

Marie Stopes International (MSI), provides family planning, safe abortion and reproductive healthcare services in some of the world's poorest countries, including Afghanistan.

Heifer International, provides livestock and training to people in developing countries, helping families improve their nutrition and generate income in sustainable ways.

Kiva.org, which allows you to make loans to people trying to start businesses in the developing world.

Mayhew International, which is helping stray, neglected and abused animals in Afghanistan, Romania, India and other countries where dogs and cats are treated horribly. They don't just try to help animals; they try to educate the general public and train aspiring vets.

Perros Project, which is helping stray, neglected and abused animals in Peru, where dogs and cats are treated horribly. If you have construction or vet training, and are self-funded, you can volunteer with them as well.

PeaceCorps Member projects, allowing you to support Peacecorps member projects in the developing world directly, like community ecotourism training, or creating a rural computer lab.

Women of Uganda Network (WOUGNET), a non-governmental organization that supports many different women’s organizations in Uganda to develop the use of information and communication technologies (ICTs) among women as tools to share information and address issues collectively.

MercyCorps, which helps both after disasters, not just with immediate help but with sustainability projects as well.

• What is the most important global development issue today?
• Youth across America are contributing their time and talents to solving global issues. How are you contributing to global change?
• How can USAID better engage American youth in development activities?

Also see

Finding Community Service and Volunteering for Teens.

Advice for Volunteer Groups / Group Volunteering.

Groups for Atheist and Secular Volunteers

Fund Raising For a Cause or Organization

How you can advocate for an issue important to you

Ideas for Leadership Volunteering Activities
These are more than just do-it-yourself volunteering - these are ideas to create or lead a sustainable, lasting benefit to a community, recruiting others to help and to have a leadership role as a volunteer. These can also be activities for the Girl Scouts Gold Award, the Duke of Edinburgh's Award (U.K.), a mitzvah project, or even scholarship consideration.

Ideas for Creating Your Own Volunteering Activity.

Volunteering with Seniors.

Advice for family volunteering - volunteering by families with children and, related, advice for teaching children compassion & understanding instead of pity with regard to poverty.

Donating Things Instead of Cash or Time (In-Kind Contributions)

Creating or Holding a Successful Community Event or Fund Raising Event.

How to Find Volunteering Opportunities, a resource for adults who want to volunteer

Using Your Business Skills for Good - Volunteering Your Business Management Skills, to help people starting or running small businesses / micro enterprises, to help people building businesses in high-poverty areas, and to help people entering or re-entering the work force.

Volunteering In Pursuit of a Medical, Veterinary or Social Work degree / career

Volunteering To Help After Major Disasters.

Volunteering with organizations that help animals and wildlife.

Details on how to quickly fill a community service obligation from a court or school.

Ideas for Funding Your Volunteering Abroad Trip.

12 Reasons Not to Volunteer Abroad
These are the most common reasons people say they want to volunteer abroad. And they are not good reasons. In fact, they often hurt people and animals in other countries, rather than helping.

Tax credits for volunteering (for residents of the USA)

How to complain about your volunteering experience.

Details on volunteering abroad (volunteering internationally).

Careers Working With Animals (for the benefit of animals)

The Last Virtual Volunteering Guidebook,  available for purchase as a paperback and an ebook from Energize, Inc.
or as a paperback from Amazon or as a Kindle book from Amazon.
This book is for both organizations new to virtual volunteering, as well as for organizations already involving online volunteers who want to improve or expand their programs. The last chapter of the book is especially for online volunteers themselves.

Other suggested books:  

Hoping to Help: The Promises and Pitfalls of Global Health Volunteering (The Culture and Politics of Health Care Work)  

Make a Difference: A Guidebook for Person-Centred Direct Support

Exploring Leadership: For College Students Who Want to Make a Difference