You need to describe your knee pain better.
What makes it worse, what makes it better, is there a clicking or poping, is your range of motion limited, do you have pain on full flexion only or at other times? does your knee swell. did you have a known injury to the knee (acute trauma?) where about in your knee is the pain? have you had a growth spurt recently.
answer these, and I will have a better response for you.
but...Based on your age alone, its quite possible its called Osgood-Schlatter (typically refereed to as growing pains). Generally, you want to focus on strengthening your quadriceps and improving flexibility of your hamstring and quads. Dont exercise if it hurts too much. Ice it daily. and dont be afraid to rest it. pushing through the pain will only make Osgood-Schlatter (if thats what it is) worse.