Hi Keith, the best thing to do is to think of your food not as a 'diet' so that you can lose weight - I think that often puts too much pressure on people and makes them feel as though they are having to give things up all the time. That isn't sustainable long-term, so what you really need to do is to re-educate yourself about food and learn how to eat healthily rather than specifically to lose weight. If you can do that you will enjoy your food more and the weight will come off, I'm sure.
A good way to start is with a food diary. Record everything you eat - and I mean everything! Include the breakdown if you can, like how much carbohydrate, fat and calories are in the food. This will give you an excellent picture of what you are actually eating - studies have shown that if we don't write it down and are asked later we 'forget' quite a lot of what we have actually consumed. One area that a lot of people find benefits them is by paying attention to portion size - it might be something as simple as eating from a smaller plate that can show results!
Once you have your food diary, you can ask your GP to refer you to a dietician, who will then be armed with all the information they need to suggest changes that will improve your chances of losing weight.
As for exercise, again your GP might be able to refer you to a gym or physiotherapist who can help you with exercises that won't involve problems with your knees.
Hope this helps, do come back to us if you have any other questions. 🙂