Okay, well the fastest way to success will be through nutrition changes. The second best option would be to focus on total body training combined with cardiovascular training. Ab-specific exercises will do very little to help. Such exercises will mainly strengthen the abs. They key is to reduce the amount of fat around the midsection, which can be done best by changes in the diet.
Competitive bodybuilders can go from being overweight to single-digit bodyfat percentages within 30 - 60 days of regimented dieting and exercise. While this is an extreme example, it shows how nutrition is the largest contributor to the lean physical appearance.