Hey, k187 and welcome to DF...I'm not even slightly qualified to answer this, but I used to have the same problem, in conjunction with really weak ankles, and found that there were lots of exercises to strengthen those areas, but it depends on whether you want exercises that you can do on your own or at a gym.
At a gym, there are endless possibilities and better-trained people to explain their gadgetry, but but if you're after do-it-yourself strength training, maybe you could try hamstring curls for your hammys and squats for your quads.
Just be careful of your knees though and be sure to check with someone qualified before undertaking the exercises. Everybody's body is different! Personally, squats kill my knees and I avoid them like the plague!