Raise money for a charity. Car washes are people did it in my high school. Or selling food/snacks a lunch or after school. Don’t offer to do people’s homework for them or anything… don’t taint your good intentions with bad things like that. You can also try to convince your school to give a certain percentage of prom ticket (or whatever) profits to a charity.
Volunteer at… any place that takes volunteers. Be careful if you go somewhere dangerous. My sister goes to a soup kitchen in a dangerous(ish) part of town but only when my uncle is with her.
Start a program to help your school become more eco-friendly. Everything from using eco-friendly light bulbs to finding ways to conserve AC/heating, etc… If you do want to do this, you’ll need to read up on various ways to make this work. There’s a lot out there though.
Organize your friends to bike somewhere and do a cleanup. No idea where you live, but a beach or a park, etc…
Habitat for humanity is pretty cool.
If you’re more of a stay at home kind of person, start a website that raises awareness and/or money for an issue you care about. Email it to your friends and family. Careful though, this is a tough one to get working since a website of this type is usually only useful once you have a lot of eyeballs on it.
The most important thing is to pick a cause you really care about. If you do that, you’ll be committed to it and you’ll find a way to help the cause. Even if you have no idea what to do, send an email to someone who works in it. Most of these people love having teenagers who actually care about something involved. Also, try to start with something small that you can succeed at. The worst thing is to get discouraged early on and give up. Remember, every little bit helps.