Easy way to tone body parts?

I'm 16, a size 12 and eat like a pig! I have a very high metabolism fortunately, but I feel as I'm getting older it's deteriorating. I've gotten into such a bad habit with the way I eat so won't be able to change my diet easily because i've tied many times before! I've noticed my upper arms and stomach getting larger. Are there any VERY easy and effective exercises I can do. ( I can't do crunches or sit ups properly because of a bad back) thank you!