My teenager is complaining of knee pain when running and jumping but not at other times. Next steps?
D/C running& jumping. At least until you have figured out why it hurts stop doing the things that cause pain. Maybe it is a simple problem that requires only some tincture of time, but maybe iris a sign of more serious damage that requires some professional attention. If a couple of days of going light on it does not help, get it examined.
Find cause. Knee pain with activity can be caused by a variety of intrinsic knee problems as well as pain stemming from outside of the knee. Getting a formal evaluation by a physician trained in musculoskeletal medicine/sports, pain, or orthopedics is a good idea!
Don't do that ;-) Short answer is to not strain yourself in that way. While there are self aid things you can do such as antiinflamatory medication and a knee bandage, i'd strongly recommend a medical exam. See your pcp and perhaps an orthopedist.