First of all start eating healthy and less, around 1700 calories daily (for more info contact a nutritionist) please don't consume less than 1500/day. You could try jogging but if that's too intense for your knee a-45-minutes walk 4-5 times a week is proven to promote weight loss. Also you can look for exercises like crunches, curls and leg raises that won't make your knee worse. You can also follow my blog on tumblr for more inspiration and tips :) feel free to ask anything. Mariloupsi.tumblr.com
Eat healthy foods, watch calorie intake and do what exercise you can. You don't have to extensive exercise; walking is good, if you can do it. Even chair exercises will help; check out YouTube for videos of chair exercises.
I know exactly how you feel. I have been up to 310 lbs. Compulsive overeater! Food solved everything for me. So I thought. Food was my choice of drug especially sugar. A friend introduced to Overeaters Anonymous. I also work the OA HOW program. I go to face to face meetings as well as phone meetings. There is also a book available on the Mayo Clinic website that is excellent. Hope this helps you.