My room was covered in posters of basketball players, because I was really obsessed with basketball. And where there weren't those, there were posters of horses, and a couple of other animals. And a few random other things. There was barely any wall visible (or ceiling...) I had a tall bookshelf with tons of books on it, and also other things like any trophies, some pictures in frames - my dog, my closest friends. Some candles. There were a lot of stuffed animals everywhere, too. The end of my bed, on this other thing next to my bookshelf, on the floor. I had a desk that I never used for anything, but it was covered in stuff all the time. Usually markers and pens. And I had a huge stack of notebooks and magazines between the desk and my bed. I had a closet & a dresser, though my clothes were also usually all over my floor. And I had a stereo, an alarm clock, cds (and tapes!), and a collection of what I called 'breakable things'. Little animal figurines or whatever they're called. I don't think my room was too out of the ordinary.
But it all depends on interests. I might've not had posters of horses and basketball players; if I were interested in ballet, maybe I'd have posters of ballerinas. Some people don't like posters at all. Or would have posters of people they thought were hot. One of my cousins had all his walls plastered with posters of bicycles, women in swimsuits, and cars.
Some people get to have things like computers or TVs in their rooms, others don't (I didn't). People who don't like to read won't have books. My sister had pictures of herself and her friends all over her walls, and she had a lot of space dedicated to makeup on her shelves. She was really into photography for a while, and you could tell by her room.
The point of my rambling is, you're really going to have to take your character's interests and personality into account. There is no one typical teen room, even if your teenager is 'cool'.