If you really wanna (want to) keep fit you should do sport and eat healthy food.
You should stop eating a lot of unhealthy food like fast food, sweets, crisps and stuff like that. The best things to keep fit or become thiner are brocсoli and oatmeal. You should eat more protein.
The worst thing you can do ro become thiner is not eating at all because your body will try to take fat fron everything so after that diet you might become fatter and you can destroy your heakth.
So the best thing, ad it was said in the begining - healthy food and sport.