ZF, sore knees are hard as it rules out most cardio type exercise. If you can get to a pool then swimming is good. Or light weights with lots of reps will get your HR up too. Bicep curls, tricep dips, shoulder presses and standing rows etc. You should be able to do some ab stuff too without putting pressure on your knee. Food really does make the biggest difference to weightloss though, but doing some token exercise is important so that when your knee is feeling OK then you will still be in a routine that includes time for exercise.