If the "calories you can eat" is at zero you have a data entry issue someplace. Check to insure you have setup your profile completely and correctly. Then recheck to make sure you have the weight goal date set appropriately. You should be trying to lose no more than two pounds per week, which is equal to 1000 calories per day.
I've read it somewhere that it's not advisable for women to go below 1200 calories per day and 1500 per men. If your daily basal metabolic rate was 2200 then you could safely restrict calories to 1200 and lose 2 lbs. per week. If you your BMR is, lets say, only 1700 calories per day then a restriction of only 500 would put you at a consumption of 1200 calories and this gives you a theoretical loss of 1 lb. per week.
All of the factors that determine your BMR are the things you input on your profile.
There are many online BMR and RMR calculators. Try reading this page
. It may help you to understand the math.