how do you build strength in your joints...specifically your knees?
Fattest Thin Man
Squats, riding, running, jumping rope. Any of these will increase the muscle strength around the knee. Even walking is a good start if you're out of shape.
The only tricky part is building strength quickly without injuring yourself. If you're old and/or haven't been execrcising for a long time, start off slow. Once you injure yourself, it sets you back for a long time, so it's much better to err on the conservative side.
Stand on one leg with a slight bend to the knee and have someone throw you a ball at various angles so that you have to twist and turn a bit to catch it. Or you can bounce it off the wall by yourself. The idea is to remain balanced at all times. If you have access to a trampoline, you can do it on that too. My therapist had me do this last fall as part of my rehab for a broken knee. Talk about losing all your strength.