Problems while working out abdomen muscles?

I’m a 15 year old guy and I think I have problem working out my abdomens. Out of my friends, I have the most stomach fat and they say that I cant get a six pack while they dont have a six pack but they dont have a blob there either, when I flex my stomach a huge one blob thing shows up, but out of everyone I know, I happen to have the strongest quads and calves, also, I have pretty big shoulders biceps, pectorals and pretty big triceps as well. I like to eat alot, and often overindulge, when I try to workout my abdomen muscles with sit ups or any other exercise then I end up using some other muscles like my quads or chest, how do I workout my abs and is it my eating habits that are preventing me from having a 6-pac?