The Teen Health and Wellness database is a good resource for lots of health issues that affect teens. This article is on violence in the media. It is several pages long, so be sure to look at the table of contents for the article on the left sidebar.
This article, from the American Psychological Association, presents the official view of that organization on the effects of violent media on children and teens.
Type "violence in the media" on YouTube and you get lots of videos from all sides of the issue. Not all will be relevant to your assignment. Watch carefully and apply PAARC to each video, especially the P for purpose and the A for authority.