I had a knee injury a while back and can no longer do many forms of exercise, like running or martial arts. Kettlebells have been the solution for me. Ideally you want hands-on instruction so as not to cause injury. They have helped me lose a stone so far (along with watching my eating), and I don't even have to go to the gym.
As for diet, try a calorie deficit of 500-1000 calories, but no more. The best diet plan is one that is balanced and one you can stick to. Remember to eat your fruit and veg, drink plenty of water, and don't forget your cheat days.
"If beings knew, as I know, the results of giving & sharing, they would not eat without having given, nor would the stain of miserliness overcome their minds. Even if it were their last bite, their last mouthful, they would not eat without having shared."