"Speak up for animals in need." Kindnews.org is a site from the Humane Society of the United States that is dedicated to informing kids about what they can do to help animals. On this site you will find information about how to help and stop animal cruelty, and even stories about how other kids have made an impact!
Katherine Noyes of Charity Guide gives a great explanation of why most animal experts will tell you that keeping your cat inside is the very best thing you can do for him or her, and for the birds and other wildlife in your neighborhood.
Write It. Learn It. Pledge It. Enjoy It. Fundraise It. Share It. This site, Kid's Planet, helps kids learn about endangered species and how they can defend wildlife. From learning how to make a pine cone bird feeder, to doing a fundraiser to help raise money, this site gives kids the tools they will need to start helping to defend wildlife and promote conservation!
Idealist.org is a fantastic site for finding opportunities to dive in and make a difference! They have a good set of listings for people living in different parts of the world, too. To find volunteer opportunities helping animals, scroll down to Wildlife and Animal Welfare under Area of Focus.
This is one good way to find opportunities to volunteer to help animals, especially for young people. Check "I want to volunteer", enter your zip code, type "Animals" into the keyword box and, if it applies, check off "Kids welcome" or "Teens welcome" and voila!
This cool program, run by the National Wildlife Federation in partnership with the USGS, makes it easy for anybody to help scientists collect information about the numbers and species of frogs and toads in the US.
Servnet.org is a great place-- for young people especially-- to find volunteer opportunities (and events) posted by local nonprofits. You search based on location, "Causes Served" (check the "Animals" box) and your "Volunteer Age Range".
For four days every February you can help scientists monitor the bird population of North America by counting all the birds you see in your neighborhood.