There's a mathmatical way to find out your Basal Metabolic Rate (BMR). Unfortunately, I am not one of the most mathmatically inclined people around! Thank goodness I did find one BMR Calculator - thanks to susanje for posting the link:
So if you are not very active, you would figure out your calories by taking 1.3 times your BMR and subtracting 500 calories per day for each pound you want to lose per week, without going too far under BMR (the author of the website where the calculator is located says you should not go under the equivalent of 10 calories per pound of LEAN bodyweight, otherwise you won't be getting in enough calories. Anyway, it's all written up there at the above link! Hope that helps out!