Changing the world is an overwhelming undertaking! If you shy away from the idea of being a full-time activist, or need immediate satisfaction, here are a few super-easy ideas you can do from your computer that take less than 5 minutes. "If I had a penny for every time you seached something on google...."
Use this Search engine instead of regular Google. Ethicle is a search-engine powered by Google. Every single time you search for something, 1¢ is dontated to the charity you choose from the site. Mine goes to greenpeace and the WWF. This is a no brainer, you don't even have to think about it. Simply change your computer's homepage to Ethicle, and go about your usual business.
Amnesty International has a page called "Take Action Online" which provides easy ways to get involved and help out with human rights issues around the world. Only a few clicks away.
Farm Sanctuary has a "Get Involved" page where you can click on action alerts to help end farm animal abuse like banning Foie Gras production in New York State. PETA also has an action center with new action alerts updated daily, like Pink's campaign to end Horse Drawn Carriages in New York City.
A great resource for more easy ways to get involved in environmentalism, animal advocacy, and social justice is THE LAZY ACTIVIST.