The tendon under my kneecap has been hurting lately the pain goes down to my shin, my knee get swollen and makes my whole leg ache. how to help?

Chondromalacia: This is very common and is also one of the easiest knee problems to heal. I would google the title of this answer and see what you can do at home to get better. Or go see your family doctor to see if physical therapy is indicated.

Osgood-schlatter: Disease is the name of a knee problem that teenage boys get in just the place you describe. It will cause a swelling and tenderness in the bump below the knee where the big tendon from the knee joins the lower leg. It is made worse with exercise and kneeling. It is not serious and every one grows out of it. Ice, stretching, rest and things like Motrin can help. Check with your parents to see md.