Knee injury diagnosis help..?

I was at a tournament this past weekend and landed wrong in volleyball causing my knee to completely give out. I have not had any previous knee or ankle injuries before and am 16 years old. I cant walk on it because it feels like it will constantly give out on me. There is mild swelling at the top of my knee, but it isn't very painful. It is not sore if I poke my knee or apply pressure to it. I just feel a lot of pressure in my knee when I try to stand on it. I feel more pain if I straighten my knee all of the way or bend it more than slightly, yet that pain is still very mild. I am going to the doctor tomorrow, but any ideas on what could possibly be the problem?