Here's another way to look at it. You need to subtract 500 calories per day from what you normally have been eating. The best way would be to forfeit 250 calories of food and do some exercise that would burn off 250 more calories. For example if you drink canned soft drinks they are about 140 cals each so you could eliminate those, or a 140lb. woman doing 20 mins. of yoga would burn an estimated 112 calories.
Here are some good websites (in addition to 3FC, of course!) to help you get started.
This web site helps you figure how many calories you need:
http://www.bmi-calculator.net/bmr-calculator/
Many of us use
http://www.fitday.com/WebFit/Index.html just to get an idea of how many calories we are taking in.
http://www.healthyweightforum.org
This site is a peer education based forum offering support, motivation, recipes and fellowship for those interested in reaching and maintaining a healthy weight.
This one gives you an idea of how many calories an exercise will burn off:
http://www.caloriecontrol.org/exercalc.html
I hope all this helps. Good luck.