Recommendations With Little Impact on Joints
Biking: If you have access to a stationary bike or you have an actual bike, this is a good exercise for you. There isn't a lot of stress on the joints and you can go at your own pace (although going fast is recommended). If you have access to a Spin class also, I would recommend it as you would have a lot of fun while shedding pounds.
Swimming: This is another awesome option if you have access to a pool and can swim (or are interested in swimming). There's minimal impact on joints, works your entire body, and you'll have fun in the water.
(Power) Walking: While running can place stress on joints (especially when done improperly), power walking doesn't really place much stress on the joints. That being said, you'll have to make walking a practice in order to see a significant impact on your weight-loss goals. Walking doesn't burn much calories; however, combining the practice with an effective dietary program will really help you achieve your goals.
Yoga or Pilates: These exercises place less stress on joints and are effective exercises. Try them and see if you enjoy them.
Tai Chi: This isn't primarily an exercise; however, its movements do not place much stress on joints and seriously, it's just fun to do. If in doubt, google the benefits of Tai Chi.
Be more active: The more active you are, the better your joints will become. So, walk as much as feasibly possible, stand up as much as feasibly possible, etc.
Stretching: Stretching is good for you, especially after periods of immobility. Do it regularly and you'll notice how limber your body begins to feel.
I'm pretty sure there are more options; however, there's no need to exhaust the whole list to start something.