My knee clicks too. It is called crepitus. You can look up the causes on google. There are a couple things that can do it. Mine is just a ligament popping over the bone, similar to if you cracked your fingers. For me, it is not a terrible thing, and I shouldnt really worry about it. It can cause arthritus later on in my life, but other than surgery there isnt a way to fix it. Mine is not where it needs that. I keep saying 'mine' and 'for me' because it can be caused my a number of different things, and therefore can create different problems depending on its source. If you are worried about it, I would see an orthopedic doctor.
I hope this helps!
PS I have found as the muscles in my legs are getting stronger my knee is cracking less. I believe that the muscles are taking over for the ligament. Your body always reacts better when it is in shape.
"Your past does not equal, nor does it dictate, your future."