I need help on how to loose weight?

[Maddi]

I am 15 I have arthritis in my knees hands and jaw and I on prednisone (steroids) to help get my arthritis under control and I am like really upset about my weight and dont know what to do ive tried diets and everything but its really hard on the steriods and I know I make excuses but I was just wondering can anyone like give me any exercise to do or any good diets that actually keep the weight off once u stop them I would be really grateful also something that's cheap not like the venus factor because I am 15 and dont want to ask mum for money as she is already struggling