Abs are made in the kitchen, and sculpted in the gym.
The first step you need to take is determining the calories you consume. Your initial goal should be somewhere around 13-15 calories per pound (so long as you workout 3-5 days per week). Then lower it to 9-10 calories per pound. But you will need to cycle as your body fights against you and the low calorie intake. Eat more on days you workout heavy and less on days you don't.
Then you need to pump up your protein intake. Protein will help boost your metabolism and keep you from losing muscle mass while on a low calorie diet.
Next, carbohydrates. Don't make them the enemy, but know when and how much you should be eating. You need carbs for energy, but not all carbs are created equal. Your goal should be for 'good' carbs. You'll need to cycle your diet with these, aim to have low levels of carbs but if it's too low for too long you'll deplete energy, have intense cravings, and your resting metabolic rate will drop. About once per month you'll want to have a carb-overload and eat a huge amount of carbs (but also low in fat) to keep your metabolism firing on all cylinders. Also make sure to keep your protein intake about the same to not throw off your muscles.
Finally, fat. The fats you consume should come from healthy sources and be low in grams. Nuts, seeds, olive oil, or fish oil are some examples. Your diet should not exceed 15% of calories from fat. But for a while you'll want that number lower to reveal your abs. Make sure you avoid fatty foods immediately before or after a workout, as it will only hinder your progress since it slows down the digestion process. You should stick to protein and carbs.
So let's do some math
Keep in mind:
one gram of protein = 4 calories
one gram of carbs = 4 calories
one gram of fat = 9 calories
For example, a 125 pound woman who exercises 3-5 days per week would need to consume between 1125 and 1625 calories. Of those calories, 150 to 190 grams should come from protein. So if we stick to the low end for an intense program, let's estimate a 125 pound woman whose goal is 1200 calories. She should eat 600 calories of protein, and the remaining 600 should be split between carbs and fat. Aim for 25-50 grams of carbs (so around 120 calories of carbs), and the remaining from healthy fats.
Other things to keep in mind:
Your sodium intake. Too much sodium will lead to bloating. Get enough sleep, your body needs rest to recover and to kick carb cravings. And be dedicated to the plan.
While what you do in the gym does matter and will have an effect on your physique, but it will be wasted if you aren't following a healthy, nutrient-rich diet. Remember it's a lifestyle change, not a fad or a trend. You have to be 100% dedicated to making the changes to reach your goals.
