Some of the ways people can help are picking up trash, removing old cars and appliances, planting trees to control runoff and erosion, and taking water samples to identify pollutants.
Most areas have programs to help citizens (including kids) clean up their local streams and keep them that way. Your town, county, or state may call its program Stream Team, Adopt-A-Stream, Operation Clean Stream, or just Stream Cleanup, but you will probably find it right away with a Web search on the name of your town plus "stream cleanup."
Although many of these activities are for adults, there are plenty of ways kids can help. The more kids get involved, the better, so you might want to talk to your county parks and recreation service, teacher, scout leader, or other group leader to organize a project just for you and your friends.
Cleaning up your favorite stream can be hard work, but you'll be helping the fish and other animals that live there, as well as making it a nicer place for humans to visit.