I actually started my weight loss a number of years ago at 343 lb. Exercise was very difficult for me at that weight. I could walk on the treadmill and swim - and I did for a while - but any exercise was exhausting in a way that it's not when you are thinner.
My answer to that was not exercising. I ended up losing 100lb without exercise. As long as I was eating around 1500 calories a day, weight loss wasn't a problem at all.
In the two years since I lost that weight I ended up gaining back about 50lb. In part I think that's because I didn't replace my negative coping strategies (eating) with more positive ones (exercise). Once I hit 250lb I plan on starting exercising as a way of dealing with stress to prevent regain and not as a tool for weight loss.
So I would suggest that you be kind to yourself. If your knees hurt when you exercise and you're able to stay on a weight loss plan, consider not exercising for a while. If you enjoy exercise and think it's great fun, follow the great suggestions you're getting here. The only right answer is the answer that will allow you to continue with and maintain your weight loss.