Sit ups help?

I know for some people sit ups come so naturally ( or seems like it ) . But for me it's impossible.I can't even do one.

I had never done or tried to do a sit up/push up till a few months ago. I can manage with pushups now but still can't do sit ups. I get as far as a crunch and no further without the support of my hands.

Any advices on how to strengthen abs muscles enough to do a complete push up? One should do pilates or more crunches?