I have a bad left knee, I wear a brace every time I work out on both knees. The bad one and the "good" one. I maintain form. I haven't had any problems with lunges/squats/etc. If I were you I'd start with upper body movements. Maybe a basic lunge or squat after you buy some knee braces with your own body weight. If it hurts, stop. You can always consult your doc but they would likely tell you the exact same thing. Unless you are on the verge of needing a knee replacement.
Good luck! I'm so happy I started weight lifting. It's done far more for me than the tons of cardio I was doing.