The three muscles I'll refer to in this are rectus abdominis (the six-pack), external obliques and internal obliques. Rectus abdominis run from your pubic region up to the base of your sternum, while your internal and external obliques run diagonally down toward your hip and up toward your sternum from your sides (around navel height). For building our six-pack we often use exercises such as crunches in all different shapes or forms and for the obliques we often do crunches with twists or bend our upper body side-ways. Doing these exercises with weights or with enough frequency and intensity can cause muscle hypertrophy, which just means muscle growth. If you have a little covering your six-pack, adding significant muscle bulk without losing the fat, will make your tummy protrude even more. And adding bulk to your internal and external obliques will cause your midsection to widen, and if you want it as narrow as your genes allow you do, this is not optimal.
To get your midsection tiny and trim, you need to reduce your body fat levels - nothing else will work. But if you want to strengthen it (which you should) to also make it physically firmer, look to held abdominal and core exercises. The simplest variation is the plank (can also be called a hover) and it can also be done both flat and on your side to target either your front or your sides. So, avoid adding weights to your abdominal and oblique exercises, and keep your eating clean and spot on. You could even build your gluteus muscles (bum muscle) or widen your shoulders slightly to make your waist appear smaller.