What should I be eating?

On my usual day, I would eat Coco Pops, two McVities Choc-chip cookies, three slices of ham, a pizza with dessert and that's usually about it. On weekends, I would have some sort of meat with potatoes and occasionally some strawberries, grapes, a banana, an orange or apple. I would also drink a litre of water and a litre of milk. This usually adds up to 800 to 900. I have recently started counting calories now and have now only realised that I'm not eating properly. I'm not sure what snacks I should be eating to add more calories. I am 13, weigh 10 stone and 7 pounds and I'm 5' 3''. I also exercise on the treadmill, at the pool an do sit ups.