To improve your overall appearance, build your core muscles and tighten your midsection you need a good ab exercise routine.  To get the best results, you should target all of your abdominal muscle groups – upper abs, lower abs and obliques.  Perform this routine at least three days a week, but you may do it every day.  These exercises are very good for giving you a slender middle and a flat stomach, they will not necessarily give you the big abdominals seen on bodybuilders – only an intense workout done only a few days a week will give you that.

Any ab workout will work best when combined with a cardiovascular and weight lifting routine.  Read reviews here of programs that incoroporate all three elements.

Complete 3 sets of as many repetitions as you can perform – be sure to use good form.

1. Sit Ups – works mostly lower abs, but also upper abs and a little of obliques

2. Leg Raises – works mostly lower abs, but also upper abs and slightly obliques

3. Crunch – works mostly upper abs, also lower back
Lifting your shoulders and legs at the same time – combining a leg raise and a crunch – is a more difficult move you can try. Don't forget to maintain control, lift only your shoulders off the floor and keep tension only on your abdominals.

4. Knee Ins – works mostly lower abs and also upper abs and obliques.
Make this move more advanced by holding a weight between your feet or combining the move with a crunch. 

For a full description of how to complete these exercises, read this article.  If you are an absolute beginner, start with only one set of as many repetitions as you can complete.

Try using bodyweight exercises to Work several muscles all at the same time and build your entire core.  Bodyweight exercises work multiple body parts, including abdominals, so they burn fat faster.  You can include weights with some of the moves – hold dumbbells either at your side or crossed in front of your chest in a squat or lunge, for example. Some good bodyweight exercises to use are push ups, the plank, the side plank, the quadraped crunch, the wall squat and jumping jacks. You can find descriptions of these bodyweight exercises in Body Toning Exercises.