It is a “Catch 22”; lose weight and your knees will feel better, but to lose weight you must exercise and that makes your knees ache. The heavier you are, the more stress is placed on the knee joints as you move. But move you must to solve the weight problem.
The best solution?
Start with the knee exercises and stretches that I recommend on this site. They won’t help you lose weight, but they will help to balance the muscles around the knees and will, ultimately, allow you to participate in any cardiovascular type of exercise (which will help you lose weight).
Try aerobic exercises that don’t place stress on the knees. Try swimming or water aerobics to start. Then move to a stationary bike (just make sure to position your seat up high enough so that your knees are relatively straight as they complete each cycle.) The Elliptical machine is a good next step as it affords a great workout with little pounding or stress on the knee joints. And by this time, you have probably lost enough weight that walking or even running on a treadmill won’t aggravate your knees.
Remember that for every 11 lbs. you lose, your knee symptoms will be 50% better! So find a low impact form of activity that works best for you and keep on moving!