The good news... You already have a "six pack". That is, the big rectus abdominus muscle that runs from sternum to pubis and has six "lobes".
In order for this to "show"....You need to do two things. First, develop the muscle with any standard suite of abdominal exercises.
There are dozens. Don't neglect the side "oblique" muscles.
Second... You must reduce total body fat to the point that you can see the muscular definition.
That may be more difficult. If you are already relatively thin... No problem.
However, lots of people have a genetic tendency to deposit fat on/around the abdomen.
If this is the case, you may need to reduce overall body fat to a very low figure for your "six pack" to become visible.
What i did was i did 200 at the min sit ups per day sets of 50 less then month had a great 6 pack
look up a video called ab ripper x best workout ive ever don't I did it like 5 days a wk and still continue to also depends on your body fat percentage aswell. don't ever worry about training abs too hard so long as your safe the abdominal area is a strong muscle and should be worked on everyday because its use to all the grind you put on it however if your a beginner I suggest doing that workout only 3 times a wk alongwith another routine
Try buying the work out DVD px90 or insanity either one will work the hell out of you!