To elaborate, one must also find out how many
calories one burns just by existing, or at least an approximation thereof (the only way to find out the real number is to spend a day or more in a special booth that measures this with relation to how your body uses
oxygen - obviously this isn't much of an option, so data has been generalised from many of these measurements based on
sex,
age, and
weight.) This number is also known as the
Basal Metabolic Rate or BMR. This is the number of calories you would burn if you lied in bed all day and did not move at all, just carried on basic physical functions like breathing.
You also have to factor in the amount of calories you burn in your daily activities. You'd want an average estimate so you don't go look at the website
coby posted to find out how many calories walking to the
water cooler burned. This can be accomplished by getting an estimate based on your
lifestyle. Fitday.com will give you one if you get one of their free accounts (use a fake or spam-dedicated email address to avoid email from them, it makes no difference - for the record,
I am not affiliated.) Cyberdiet.com can also give you an estimate.
Work out how many calories you'd need to eat to maintain your current weight and check this with real experience - eat an average of that many calories every day for a week or two and make sure your weight is the same as it was when you began.
There are other things to take into account:
- What to eat. While, as George Dorn pointed out, low-carbohydrate and low-fat diets don't work inherently, having a diet that excludes calorie-dense foods is a better bet than one that doesn't. Yes, you can lose weight on butter alone, but you'd eat such a physically small amount that you wouldn't feel full (most people need at least around 3lbs of food per day to feel full.) Constantly not feeling full will lower your chances for success. It doesn't matter where your calories come from, and calories are the only thing that affect whether you'll lose weight (not fat grams, sugar grams, etc.), but you should be sensible about these things (fat has more calories than protein and carbohydrates, for example, because it has less water per gram).
- Refeeding. While dieting, you should refeed on occasion, to increase the levels of leptin, an anti-starvation hormone, which will drop with caloric deficit (when caused by diet, that is). Refeeding involves eating at or slightly above maintenance (the amount of calories you would have to eat to maintain your weight) occasionally. How often depends on how many pounds of fat one has to lose. The more extra fat you have, the less often you need to refeed. An obese person could get away with only doing it once every two weeks (once a week may be a better idea for sanity's sake, depending on the individual.) When one comes closer to one's ideal weight, one needs to refeed more often to keep the weight loss going, maybe as much as every other day. This is also a great way to gradually learn how to eat for your new weight (the amount of calories it takes to maintain your weight will decrease as your weight decreases, which is why if, after losing weight, you go back to eating the amount that maintained you before, you will gain weight.)
- Don't eat too little. If you eat less calories than you consume, you will lose weight, this is simple thermodynamics, but an extreme deficit seems to not only compromise bodily fucntions, but change the ratio at which you lose lean muscle mass (normally it's 1:3 - one pound of lean muscle for every 3 pounds of fat,) which is undesirable, because it is lean mass that makes our metabolism what it is, and the more muscle we have, the more calories we burn, this leads into...
- Weight training. Yes, this is for *everyone*. Men, women, young, old, thin, fat, it makes no difference, weight training benefits everyone. The more muscle mass we have, the more efficient we are at burning fat. We will lose muscle mass from dieting, it is inevitable. Luckily, newbies have the benefit of being able to burn fat and put on muscle at the same time (once you have more experience, it's no longer possible to do this, though, AFAIK, no one knows why.) So lift those weights! Don't fear the free weights! (Women who fear bulk needn't, very few women have sufficient testosterone to have massive muscles, and even those with the capability would have to work very hard to achieve bulky masculine musculature - as do men (usually).) An excellent site about weight training for women which has useful information for everyone is http://www.stumptuous.com/weights.html.