The best thing to do when trying to get abs is lots of cardio workouts. Eating all three meals is also very important and lifting light weights. You can go to http://www.wikihow.com/Get-Six-Pack-Abs for more tips.
I would say crunches and sit ups. I do 75 sit ups in the morning and 80 crunches at night everyday and more at basketball, football, and LAX practice and more every other day when I go to the gym haha.