What Causes Terrible Pain In The Knee While Doing Strenuous Exercises?

i am 21 about 5ft9 11st ish, my grandad and mom suffer with knee problems, with my right knee when i hold it up infront of me next to my other knee i cant keep it there, my left is fine but when i hold up my right it crunches and i get a terrible pain through it, when i do strenuous exercise too it goes, what shall i do?