Your muffin top could be one of three things, or a combination of them. Firstly, additional fat around your middle. Secondly, extra skin from pregnancy. Thirdly, poor abdominal muscle tone. You can't really do much about extra skin sadly, but you can work on the fat and muscle tone. To reduce fat deposits, you need to do general cardiovascular work - like riding a bike, running etc 3-5 times a week for at least 30 mins. And of course eat sensibly. Doing some abdominal exercises will increase muscle tone, but won't remove localised fat. To work on your abs, you've got three muscle groups to exercise - the rectus (sit ups/crunches), obliques (sit ups/crunches with a turn to each side) and deep transversus (abdominal bracing - needs to be taught to you by someone who knows what they are doing.

If you can find an activity that combines cardiovascular work with abdominal work and is fun, you'll be sorted! Alternatively, buy different jeans and don't stress about the muffin, just don't expose it.