everything cjohnson728 said, I would add
1. plank holds -hold yourself up on both elbows with your elbows, arms, and hands in front of you on the floor, arms bent at elbows - keep your body from head to feet as straight as possible. Start with 30 seconds or whatever you can do - work up to 2-3 minutes.
2. leg holds (variations also called lying leg lifts) - lie on your back, arms palms down on floor or mat, don't arch your back, raise your legs 6-12 inches and hold for 15 seconds for each repetition. Gradually increase repetitions. Works lower abs - the bottom two of the six pack.