Hi. I am dealing with my weight and a wreck-y foot that limits the amount of exercise I can really do.
The first thing I'd say is you need exercise to be fit as far as flexibility/muscle building, but weight-loss itself is mostly diet. And that's good and bad. It means you can't do what I used to do (before the foot drama) where I rationalized eating all sorts of stuff under the banner of how much exercise I really was doing, but also dieting is not going to stress your knee. There is no such thing as high-impact must-avoid broccoli.
What eating well is does vary for a lot of people. I find I personally do best when I limit calories but don't do "must avoid all fatty things" or "must avoid all carb-y" things. I eat chocolate and I eat butter and I eat pasta, I just eat less of them than I used to.
But because portion control is my downfall, it sounds stupid but drinking lots of water and eating lots of green vegetables is more helpful for me at least than most other things.