By the time your teen reaches age 13, he/she has probably seen around 15,000 hours of television [Source: Peaslee]. Add to that the Internet, which teens now consume in larger quantities than even television, and you have teens actively watching media as almost a full-time job - - over six hours a day on average [Source: Peaslee]. In order to limit the influence the media have, you do need to teach your teens how to watch media with a critical eye, not accept what they see as truth, and evaluate the claims made in the media. In other words, you need to make sure your teen is media literate so he/she can consume all that media exposure in a healthier way.
Since you're not going to be able to shield your teen from most media exposure, you can help him/her learn that media producers create their product with their own influences, agendas and biases. You can start the conversation with your teen by knowing (and asking) what he/she watches and reads. Then follow up by asking him/her how he/she feels after engaging in a specific media, what he/she believes the producer's goals were in creating the media product, and whether what he/she saw conforms to what he/she knows to be true or contradicts it?
Researchers have also found that the best way to teach media literacy to teens is to have teens produce their own media programs [Source: Chen]. The technology is easy enough to find (it's probably on your teen's cell phone!). Once teens start making their own editorial decisions regarding what to include and how to present information, they start understanding better how the professional media images they see went through the same process. You can do this with your teen on your own, or ask your teen's school or youth organization to run such a program.