What do I do about knees that lock up?

Ever since I was in middle school, my knees will lock up on me whenever I bend them less than 90 degrees. Usually it would happen once or twice every couple of months, but now it is happening almost every other week. My knees would lock and I wouldn't be able to move them at all, its like something got stuck in there and i cant unbend my leg. Not to mention it is very painful and if I suddenly unbend it I hear a "pop" and the pain is unbearable. After experiencing my knees locking so many times I have learned to massage the knee in order to unlock it, even though I still feel pain for the next couple of days. I have never had surgery on my knees, but sometimes they hurt when I kneel or bend down. I can also hear a creaky noise when I bend my knees.

I have no clue what this is?? Does anyone know what's going on with my knees?