You need to figure out your BMR (basal metabolic rate) and then calorie needs based on activity level, the you subtract the amount that would give you weight loss.
http://www.bmi-calculator.net/bmr-calculator/
The first page will give you your bmr, then click on the link to take you to figure our your calorie needs, then click on amount to lose weight.
Edited to add: it doesn't give good descriptions on that website for activity level. I looked at a Jillian michaels book that spent a lot of time on this math formula and I chose that my lifestyle was lightly active. But after I started I'm now exercising five days a week. So I choose not to "earn" more calories to eat based on my exercise.
Dang edited again to add: once you know your calories per day it explains that you need 7,000 less to lose 2 pounds per week. So that's 1,000 less per day.
For me the formula gave me about 2,555 calories to maintain my starting weight. So I subtracted 1,000 per day. I shoot for about 1,500. Some days happen to be a little less, some a little more.