Which are the exercises that will tighten up my stomach after having a baby?

Many. Check out "core " exercises online. Everything from crunches to leg lifts will help tighten muscles, but remember to lose the fat around the midsection you have to lose total body fat, and that often involves aerobic exercise. Spot losing fat is a myth, as are most fad diets and contraptions. Stick to the basics of good nutrition and exercise.