There is a LOT of posts about this topic, so if you do a search you should be able to keep yourself occupied for hours. But I'm happy to answer your question anyway....
The question of how to get a 6-pack is something that isn't readily agreed on in the bodybuilding community. Some people think abs need to be trained like any other muscle with heavy resistance and low reps, others think high rep ranges and no weight work best. Some think abs training is totally unnecessary. Some think once a week is sufficient, others do it every day. Like anything else in bodybuilding, you really have to experiment and figure out what works for your body.
But the bottom line is this: No matter how effective your abs training routine, it only achieves one thing: Increasing the size of the muscle fibres. If you have a layer of fat smothering your mid-section, you won't have the 6-pack.... no matter what your abs training is like. So ultimately, it comes down to diet and cardio to lose the fat on your abdominal region.... everyone already has a 6-pack, you just can't see it on most of us. Abs training is really only useful for increasing the size of the muscles so they're more defined once you lose that fat.
Having said that, this is what I do:
Hanging knee raises (4 sets, ~20 reps): Hang from the chin-ups bar, and bring your knees up until your thighs are parallel with the floor. This is your STARTING POSITION. Without swinging, bring your knees up as close to your chest as possible.... hold for a pause, and then bring your thighs back down to parallel. Your thighs don't drop below parallel with the floor. At the top of the movement, you can tilt your pelvis up for an extra contraction.... after about 10 it should start burning a bit. I superset these with hyper-extensions for lower back.
Oblique crunches (3 sets, 20-30 reps): Lay on your back with your knees bent at 90°, and then drop one leg down sideways so it's flat against the floor. Then do a twisting crunch, squeezing with your obliques at the top of the movement. Do about 20, and then change sides.
Standard crunches (3 sets, 30 reps): Standard crunches.
I only train abs once a week.... that seems enough for me. They're easily over-trained.
But again, I must stress that all the abs training in the world will be futile if you don't diet and do enough cardio to burn that fat off your mid-section (assuming you have some to begin with, but most of us do)....
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