I'm in my teens and I'm looking for some advice which could help me I've had a pain in my knees for?

That might be caused by a lot of things. Strangers on the internet have nothing to base a guess on, so you are going to have to learn a few things and figure it out by yourself. Bones are made of calcium, the pads in the joints are made of vitamin C, and muscles depend a lot on magnesium. There are a few other details involved. The important thing is that doctors are trained in medicine and surgery. If your condition involves nutrition, they have no training in that field. So you need to study it at least enough to tell a doctor what test to order.