Everyone has had some good advice, weights, ab routines, and stuff like pilates will do wonders. I do have to say though that honestly some people cannot healthily attain 6-pk abs just because of their individual fat distribution. For me personally, I do a TON of varied abwork- yoga, pilates, free weights, machines.. and I can't get ab definition. The only time I could see it was when I was really underweight, and now, even at the very bottom of my healthy weight range, I can't see a thing (other than my abs are very flat and hard to the touch- they just don't look it!)
So yeah, I don't want to bum you out because you should definitely try some different things to build up strength, I just wanted to let you know that some people just aren't genetically cut out for it.