Your abdominal muscles are the same as any other muscles in your body. They work similarly to your biceps, triceps and hamstrings, so why is it that we treat them so differently? Abs cover a large part of your body and a well defined midsection says, "I'm in shape."
Because of the way the body works, most people store a large portion of their body fat in the midsection. If you have a perfect set of abs covered by a layer of fat, no one will be impressed with your hard work. Getting abs is a two step process. Step 1: lower your body fat. Step 2: build up your abs with sit ups and crunches.
To lower your body fat, you need to take the same steps you would if you were looking to lose weight. Exercise-wise you need to engage in some sort of cardio. Cardio can be almost anything you enjoy doing.
If you want to go to the gym you can run on a treadmill, ride a bike, use the elliptical or rowing machine. There are plenty of options to choose from.
If you'd rather save some money and do cardio outside of a gym there is more than enough to keep you busy. Running, biking, walking, swimming, sports and any type of work (yard work, house work...) is good cardio as long as you do it on a regular basis at a high enough intensity. Read more on cardio guidelines.
Once you have your cardio in order, you need to begin targeting your abdominal muscles. If you have a high body fat percentage with a nice set of abdominal muscles, chances are you won't see anything impressive. On the other hand, if you have a low body fat percentage and you never work on your abs, there will be nothing to show off. You want to include your abs in your workout just as you do all your other muscles.
Don't go into the gym everyday with the goal of killing your abs. Remember that to grow or improve their appearance, muscles need time to rest and heal. If you're doing hundreds of crunches or sit ups everyday, you'll never see an improvement. Its like picking at a scab, it can't heal.
If you choose to work on your abs every time you go to the gym, don't go till failure on every set. Do 3-6 sets on a lighter weight and move on to another muscle group. If you only work on your abs once or twice a week then you can do more sets and you can do them till failure. Failure means that the last repetition you do is really the last one you could physically do. Find a weight with which you can perform between 8 and 12 reps.
Abs can't appear overnight. You need to get your exercise as well as your diet in check before you see any progress.