To determine your BMR:
Women: BMR = 655 + ( 4.35 x weight in pounds ) + ( 4.7 x height in inches ) - ( 4.7 x age in years )
To determine your total daily calorie needs, multiply your BMR by the appropriate activity factor, as follows:
1. If you are sedentary (little or no exercise) : Calorie-Calculation = BMR x 1.2
2. If you are lightly active (light exercise/sports 1-3 days/week) : Calorie-Calculation = BMR x 1.375
3. If you are moderatetely active (moderate exercise/sports 3-5 days/week) : Calorie-Calculation = BMR x 1.55
4. If you are very active (hard exercise/sports 6-7 days a week) : Calorie-Calculation = BMR x 1.725
5. If you are extra active (very hard exercise/sports & physical job or 2x training) : Calorie-Calculation = BMR x 1.9
http://www.internetfitness.com/calculators/bmr.htm
Then once you figure your AMR, you want to eat about 300 calories below that to create a calorie deficit and start to lose wieght. Eating at your AMR will just allow you maintain your weight, while eating above it will definately cause you to gain.