Abdominal muscles take the longest time to develop, require a low level of body fat to be seen and hence beginners struggle with getting them. There are three main muscles in the group of muscles that form your abdominals:

1. Transversus Abdominis - a thin muscle strip that goes horizontally across the abdomen.
2. Obliqus Abdominis - otherwise knows as your obliques, it runs diagonally along the mid-section from the lower rib cage to the pubic area.
3. Rectus Abdominis - or your abs, the large, flat wall of muscle that runs in front of your stomach, from the lower chest to the pubic bone.

Try these exercises to target the above mentioned areas:

1. Side bends - 15-20 reps, repeat 3 times. (Obliques)
2. Pelvic tilt - 15-20 reps, repeat 3 times. (Lower abdomen)
3. Crunches - 15-20 reps, repeat 3 times. (Upper abs)

Remember to schedule different days for different body parts so as to reduce the chance of getting injured and/or worn out.

Day 1: Triceps, Chest, Abs
Day 2: Quads, Forearms, Calves
Day 3: Hamstrings, Shoulders, Abs
Day 4: Biceps, Back, Abs

If you can't handle doing 3 sets of each of the exercises, start with 1 set in the first few weeks and add 1 set a week till you reach the maximum of 3. Combining your abdominal specific exercises with some form of start-stop exercises (football, basketball etc.) as well as a weight lifting program will provide the best overall results in losing body fat and showing your stomach muscles. Combine it with a good diet and you're good to go.