3 main ones you need to do.

Obliques side of your Abs

Lower Abs

Upper Abs

Now you should look up on the internet, that will give you the best Ab exercises. I sometimes do a variety of Ab exercises in one day, other times i like to just focus one day on one part for example the lower Abs.

I recommend doing.

Bicycle Crunches
Excercise Ball Crunches
Excercise Ball Oblique Twists While Holding A Weight
Exercise Ball Oblique Twists
Dumbell Side Bends
Double Leg Lifts
Hanging Leg Raises
V-Ups
Jackknifes
Captains Chair
Reverse Crunch

ETC.

Don't forget to work your lower, upper, sides, of your abs. Don't neglect any part like so many do. And remember variety is the key and keep a track of your progress, if your new to doing Ab workouts. Then i suggest 8 to 10 reps x3 would be fine.