I suggest buying The Abs Diet
Great book. If ya listen to the guy, you'll get a 6 pack in no time. I also suggest 'Essential Abs' by mens health. When i started with that book, I had no pack at all. After a month of doing it every other day, you could see my 6 pack. It has a beginner level, intermediate, and advanced. Beginner takes about 15 minutes or less, Advanced takes about 25.
Buy both of these books if you're serious about building your core, you will never be sorry you did. I've recommended both to at least 20 people.