It's not as important to work on your oblique muscles at first, but eventually you'll want to start working these too. These are the muscles to either side of your stomach. There are multiple ways to do this and anything that includes twisting your torso against a resistance counts. There are twisting machines at gyms, you can twist while you do sit-ups, you can do side bends, you can twist side to side with a medicine ball in hand, etc. Be aware though, that many beginners tend to have weak obliques compared to their abs (it simply isn't used as much in daily life) so go easy on the sides at first.