Hi Birdie,
You can sit in a chair and raise one foot up so your leg becomes straight. Lift your leg slowly and carefully over and over again. Once you can do that, add a very light weight to it and keep working your way up. The thigh muscles will strengthen and protect the knee. A stationary bike would be a good tool to use, that builds strength around the knee while also giving you some cardio benefits. Standing leg curls can help as well. I hope this helps. Thank you for asking!