What Causes Stabbing Pain In Knees?

I have really bad knees but dont know what is causing the problem. Something on the bottom side of my knee will pop out if i move my knee a certain way but it does not do it all the time. Then when it does this it feels like a knife is being stabbed in my knee. And i have to pop it back in myself and man does it hurt. And when i will be walking they will jusat give out on me and i will almost fall down. I have been dealing with this since i was 12 years old and now im 23 and it has not gotten any better. Can u help me with this please.