1. The plank.
Try and keep those stomach muscles nice and tight and maintain your body in a straight line. Try to hold it for two minutes, then switch to the next exercise.
2. Lateral plank (aka Side plank)
Try to keep that body straight again, make sure your bottom doesn't drop down towards the floor. If it's too hard to start off with, try leaning on the elbow of that arm instead. Make sure you look up towards the hand that isn't supporting you so that you don't strain your neck, and don't wear socks- you need traction. Again hold it for two minutes.