YSK if you suffer from knee weakness or pain, focus strengthening your quads (the muscles between your hip and knee on the front side of your body)

You should first get back cleared by your primary care physician if you are at high risk of injury. To strengthen this muscle group, any exercise that's responsible for bringing your leg straight from a bent position will work your quads (leg extensions, upward phase of squats, upward phase of lunges, etc). This with a combination of maintaining a healthy weight will help rid you of knee pain and discomfort. Source: ex phys major and worked in physical rehab for many years.