How to get a flat stomach?

Okay, i really want a flat stomach for the summer so i thought i would start now, i do sit ups every night, around 50 or 100, sometimes more.
I don’t really have a healthy diet because i am a very fussy eater, i dont eat nothing hardly, all i live on is chips, nuggets and sausage mainly! can anybody help me get a flat stomach without me changing my diet too much? i have cut down on junk food and snacks inbetween meals
thanks x