My left knee keeps locking up and it hurts so much, what should I do?

One of your quadriceps muscles (the rectus femoris - google it) could be the issue. This muscles runs down through your knee behind the kneecap. Don't worry about massaging your knee, try massaging your quads especially the muscle I mentioned to try and loosed it u a bit. Start massaging it daily and see if the problem eventually fixes itself. Good luck kid.