I had some problems with my knees a few years back and this is one of the moves which my physio recommended in terms of building the strength around my knees and upper leg muscles (quads). It also is a great move for improving balance.
How to: Start off in a standing position and bring your heel towards your bottom. Choose your arm position. Extended in front (harder) or crossed over your chest (easier).
Now you: Slowly squat down and use your thigh muscles to pull you up. Try to keep your knee in line with your big toe. If your knee is drifting towards your middle toe, start bringing the squat back up. Start off with 1 set of 12-15 reps and build up from there. Remember the stronger your muscles become, the deeper the movements you can make.