If you look on the wolfram alpha website and search for basal metabolic rate there is a good tool therre to calculate your daily calorie requirements
Check out http://weightloss.about.com/od/eatsmart/a/blcalintake.htm - there is lots of information there about how to do the calculations.
The number you will get will tell you how many calories you need to eat in order to MAINTAIN your weight. In order to lose, you need to cut back on your calories - about 500 calories less than you need to maintain is a good starting point, as long as you never go under 1200 calories a day - and increase your exercise so that you are also burning calories every day. As you lose weight, you need to recalculate the number of calories you should be eating every day, and when you reach your goal weight, you need to actually eat the number of calories you need for maintaining it. And, of course, continue exercising throughout! :)
I hope that is helpful.