Suggest Treatment For Knee Pain With Knee Cap Sliding Off Feeling

I am 17 and have been having problems with my knee for about a year. Every time I am active my kneecap slides out of place. It slides right back in but it makes my knee feel weak and stiff. Also at random times I'll get really sharp pains in my knee around the kneecap, and it hurts to walk when this happens. Also after I sit down for a long time, my kneecap slides out of place every step for the first few minutes. What is wrong with it?