How can I get rid of belly fat and get flat abs?

Dan =] asked:

I’m skinny but when i seat down you can see my belly, it’s nasty cuz i’m skinny. so i wanna get rid of the fat and get flat abs! I’ve been staying in the gym and doing cardio and some sit-ups. but i don’t think it’s working! How many sit-ups a day or a week do i need to do?