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.
Mini Goal #1: 199 onderland Done! 12/12/14!
Mini Goal #2 193 lower 190s. Done! 2/20/15
Mini goal #3. 186 Lost 10% of BW Done! 3/28/15
Mini goal #4. 179 170s again. Done! 5/13/15
Mini Goal #5 176 Healthy BMI Done! 6/15/15
Mini Goal #6 169 160s!! 9/1/15
Mini Goal #7 166 20% body weight 10/23/15
Ultimate goal: 20% body fat-~159 lbs 1/1/15