I'm going to start out with the bottom line, then fill in the missing pieces:
A 6-pack has everything to do with the amount of body fat you have. Whether that 6-pack is impressive or not has to do with muscle mass.
The magic threshold you want to get to is under 10% body fat. The best, most tried and true method to deal with body fat is to get your diet in order. However, there are some caveats if you want to put on muscle mass as well. I recommend checking out Lean Gains, The Warrior Diet, or something of that approach to help you balance the two.
Now, in order to get muscle mass on your body you have to do some strength training. The best bang for your buck is going to be linear progression (increasing the weight you lift every time you go to the gym), until you max out the gains from that. I've written up a good primer on strength training that summarizes a lot of the info I learned when I was just starting out. In that primer is a link to three pretty solid beginner strength training programs.
Of the three, the one that would be most compatible with fat loss would be The Average F'n Program. Despite the name, it's got a couple features that will help you as you gain strength (and muscle) while trying to cut fat:
- Only two main lifts per session and one accessory exercise. Keeps the training session short and within the optimal time to increase testosterone without increasing cortisol.
- Lower session volume helps with recovery.
With both the AFP and Starting Strength protocols, it's best to find your 5 rep max (the heaviest you can lift 5 times). Now, for a beginner the definition of a 5 rep max is when form breaks down. If that means you can only lift the bar for overhead press, so be it. Check the ego at the door, because in a very short time you will be surpassing your current abilities.