If you want to lose weight, it's important to know where to start. By figuring out how many calories your body needs for normal daily activity, you can either exercise more or eat less in order to burn fat.
Think of it in this way. Say your body burns 2,000 calories every day, just by being alive. If you ate 2,000 calories each day, your body would have no reason to burn fat. It would burn what you fed it. If on the other hand you ate 1,600 calories each day, your body would still need 400 calories to do its normal work. So it would take those 400 calories from the fat stores.
Add in a little exercise, and those fat stores melt away!
Technically, it's worth mentioning that your body does also maintain a glycogen reserve in the liver and muscles, as an emergency backup. That is primarily tapped for intense exercise.
When planning out your daily menu, it is important that you eat enough food for your body not to panic. If you eat too little, your body will think a famine is going on and cling to your fat with all its strength. It will even drop your metabolism. You need enough food so your body feels comfortable burning the fat, but not enough so that your body has "too much" coming in and stores away the excess as more fat.
I have an Online Metabolism Calculator to help you do this easily. If you want to know how it works, here are the details.
Daily calories are equal to:
Women: 655 + (4.35 x weight in lb) + (4.7 x height in inches) - (4.7 x age in years)
Men: 66 + (6.23 x weight in lb) + (12.7 x height in inches) - (6.8 x age in years)
Now you have to adjust that based on how active you are.
sedentary - multiply by 1.2
exercise 1-3 times a week - multiply by 1.375
exercise 3-5 times a week - multiply by 1.55
exercise 6-7 times a week - multiply by 1.725
exercise a lot every day - multiply by 1.9
You can see what a huge difference exercise makes! For the purpose of this formula, "exercise" means burning off 300 calories. If you are walking a quarter mile, that would not count as a full period of exercise. All exercise burn values vary based on your weight and height, but a good estimate is that walking one mile burns off 100 calories. So you would want to walk three miles a day to count it as having exercised for the day.
There are many online charts to help you determine how many calories you'll burn based on your height, weight, and your types of exercise. Take a look at your options and map out a plan.