Getting a Abdominal Six Pack might sound hard.
The concept is very simple actually, but the hard part is getting it into action.
It will going to take dedication, time and a little bit of patience to get it done, but it's very achievable.
Here's how...

Squats and Deadlifts: if you want to build really great abs, it can be nice to know what your abs actually does, what kind of function they have.
Your abs' primary job is to not curl you up into a ball.
Working together with your back muscles they make you an correct posture and stabilizes your spine.
Therefore, it's really important to work out your back as well as your abs.
So the best exercises for abs are squats and deadlifts, because they force your entire core to go into overdrive to support your spine.
Deadlifts and Squats will train your entire core to work as it should do, as it's designed to do.
These exercises will train a lot of other muscles as well, such as glutes and quads among others.

Crunches: You need to lay on the floor, preferably on a mat, keeping your arms in front of your chest.
Now bend your knees up as far as you possibly can, and do not place your hands behind your head.
If you place your hands behind your head, you can get injuries in the long run, and we don't want that.
Now start to raise your upper torso towards your knees, but remember to not lift it too far.
This is not sit-ups.
So keep in mind that you're not going to lift your entire back off the floor.
When you're up and feels that you're working your abdominal muscles, lower your back down again, but very slowly and controlled.
Do not let your head touch the ground and repeat.

Sit-Ups: Lay on the floor, in the same posture as you're doing with crunches.
Now place your hands so your fingertips barely touches your ears or keep them on your chest (crossed).
What you need to do now, is lift your torso all the way up so your lower back is not touching the floor.
When you're up, lower yourself slowly and controlled down again, and repeat.
(tip: you can try it with your feet lifted in a 90 degree angle as well)

Leg lifts: Lay on the floor with your legs straight out and hands at your sides.
Now lift your legs straight up, and keep sure you're not bending your knees at all.
Lift them up until they are at a 90 degree angle (or as close as you can get), and then lower your legs and repeat without letting your legs touch the floor.

Static Hold: Get in a push-up position, but keep your elbows on the floor instead of your hands.
Now make sure your whole body is flat/straight.
Now just keep this position as long as you can.
If you're a beginner, you should try to hold it at least 40 seconds.
Every thing over that is good, and you will probably start to shake in your whole body!

Now, if you do these, you will notice how you are using your abdominal muscles.
So for you to get a abdominal six pack as fast as possible, you can start to make up your own workouts.
Be creative! To help you out a little bit, here are some possibilities:

Use a stability ball.
Do your crunches on top of that ball in addition to ordinary crunches

Hold some weights in your hands while you're doing sit-ups to add more weight for you to lift

Do some running/sprinting.
Running/sprinting is nice for your great abs