Hey there,
You'll need to use one of the online calorie calculators: enter your age, height, weight and activity level and see what they say you need to eat to maintain your current weight. (Hint: put in 'lightly active' rather than moderately or strenuously active for a more accurate count.)
http://www.caloriecontrol.org/calcalcs.html
The general rule is that 1lb of fat equals 3500 calories. If you reduce your calories by 500/day, you should lose a pound a week. If you reduce by 1000, you should lose two pounds a week. (Or conversely, if you reduce your calories by 500 and burn 500 at the gym, then you'd probably lose 2lb a week.
These things are all pretty individual however, so you'll need to play around with your calorie counts until you find what works for your body. Starting at 1500 calories a day would probably be pretty safe.