Hi Bill again :) thanks for posting this question! Knee injuries as you may know are the most feared for any basketball player since they are our powerhouse without them we could not run, jump, twist and score! so it is extremely important to develop strength on those knees. You should not worry about the time you spent sitting down since most of us we have to sit down and remember at your age i had to sit down long time since I had to pass all my exams.
In physical strength, especially competitive athlete you need to develop strength in different ways:
One developing powerful muscles around the knees. You are looking at strength in the quadriceps, hamstrings, inner thighs and calves. All those muscles need to be trained individually and as a part of a whole body movement to improve knee mechanics.
The second is really targeting the soft tissue of the knee itself with proprioceptive exercises requiring balance, here I am sending you an exercise that helps with knee mechanics: Copy and paste: http://www.youtube.com/user/BodySmartTV#p/u/0/AFCume9pb_8
Not sure if you are doing proper and individualized athletic training when you play basketball but it is extremely important, when i was your age i used to spend 4-5 hours of physical and 4-5 hours of practice.
I hope this works..!