There are 3500 calories in a pound of fat, so conventional wisdom in calorie counting is that if you want to lose a pound a week, you create a 500 calorie deficit per day less than your resting metabolic rate (rmr) per day and you should see 1 lb a week loss. You can create the 500 calories deficit through two ways - eating 500 calories less than your rmr, exercising 500 calories more while eating the number of calories that you get when you calculate your rmr, or any combination of those two. It's up to your preferences.
Generally, no woman should eat less than 1200 calories a day because that's the minimum needed to make sure you're getting enough nutrients. I like to use Web MD's calculator here, because it calculates both your rmr AND with a few quick adjustments to the information you enter on the first page, you can tailor it to a 1 or 2 lb goal per week and choose your activity level based on how much you want to work out.
I personally like to do this calculation with 'Not Active' as my activity level because I don't reliably exercise and when I do, it's just icing on the cake
As an example to walk through the process, when I entered in my info I got an RMR of 2239 making my daily target for calories 1739. I actually only eat 1700, but that's more for the round number than anything scientific.
Hope that helps!