You are only 11 mate. You will be going through some humungus growth spurts over the nest few years and you will probably shake it off then.
You have to be honest with yourself though. What is your activity level like and how much and what kind of foods do you eat. If you sit around on the xbox most days eating chrisps, fish and chip and Harribos all the time, then that is why you would have belly fat. If you eat well and are active then the fat is natural. There is a lot of un-educated parents out there who give their kids anything they want to eat, and massive portions, but you need to educate yourself and realise when you are eating too much fat and sugar.
You are only 11 so don't worry about it. If you are eating crap and doing no activity, then change it and live healthy, get on your bike, do a sport... Doing sit ups will do nothing to get rid of belly fat. You need cardiovascular exercise to get rid of fat.