First, I think knowledge is power. So, what you would need to do is do some research and free speech and the issues that are involved. Second, along the same lines, look for examples of censorship and see how it comes about and what can be done to stop it. Third, in order to be practical, ask some teachers to help. Make it into a project and if you can get some school credit for it, even better since you would be killing two birds with one stone. Finally, I would also examine the areas that are ambiguous. In other words, study areas were some type of censorship might be necessary, such as hate crimes and hate speech.
I think that one way that teens, or any group, for that matter can protect free speech is through collective action. Any group that seeks to protect their rights and privileges must rely on their ability to outreach and form solidarity amongst one another. The form of this could be through petitions, communication, and rallying support from one another. Given the fact that so many teens are technologically savvy, the use of social networking sites, blogging, as well as texting could be another manner in which teens can find common ground over issues of censorship and protection of their rights. We certainly saw during the most recent political campaign how the young vote was vital in delivering their candidate to the Presidency. In this same manner, grass roots organization and solidarity can be met with advancements in information technology to achieve the accomplishment of specific political goals.
First of all, I don't think that there is all that much danger of censorship in the US today. However, we will go with the idea that it is a danger and teens need to do something about it.
Most censorship in the US today happens in schools or public libraries. Typically what happens is that parents complain that some book or other is not appropriate for their children to have to read. This is often because the book has too much sexual content or because it tallks about things offensive to someone's religion.
So I would say that the best thing that teens can do is emphasize to their parents that they are old enough to handle reading things that have these themes. They should tell their parents (if this comes up) that they can read things and make up their own minds and that they do not need to be protected from this kind of stuff.