Google 'TDEE calculator.'
This one looks pretty straightforward. That will tell you how many calories you need each day to maintain your current weight. Then subtract 500 calories each day to lose a pound a week, or a thousand calories a day to lose two pounds per week (but don't go below 1200, because that would make it hard for you to get enough nutrients and could hurt your metabolism).
For example, a 30-year-old 5'5" woman who weighs 200lb and gets no exercise has a TDEE of about 2000 calories per day. To lose a pound a week, she'd need to eat around 1500 calories per day.