Abs Guide - health

1)Diet
Diet is so far the most vital factor in looking "ripped." In order to show off muscles you've to get rid of the layer of fat on top of them. So if you are over-consuming or eating foods high in fat and calories, how are you going to get rid of that layer !? The important thing is to eat 4-6 small meals per day in oder to boost your metabolism and shed fat easily. The much stronger your metabolism, much energy you can gain, much workout you can do :)

2) Strength Training
The more muscle on your body, the more calories you burn just sitting around. This is because muscles require more energy to maintain from the body than fat. Developing a good amount of muscles is essential so a overall fitness plan is recommended

3)Cardio
For your type, this is least important as you are already skinny and probably don't put on muscle/fat very easily. This should be used sparingly (maybe 2 hours a week) in order to get rid of that last bit of fat.

As far as abs go, they are like any muscle. Doing high repititions (over 20) doesn't create more seperation, instead it simply increases the endurance of the muscle. You'll want to do a variation of ab workouts (find 3-4 you like) and keep the repititions from 8-15.

I personally do a series of sets that go
Hanging leg raises - 10 reps
Cable Crunch - 100 pounds - 15 reps
Jack Knife Crunch - 10 reps

I do all of these back to back with no rest, doing slow contractions and negatives to stimulate as much of the muscle as possible.

If you don't have a solid fitness program I would recommend start there and workout your way to the diet as that is pure mental, where strength training is mental, physical and more fun in my opinion

You can find a great plan at bodybuilding.com and type in beginner routines