With the New Year, millions of Americans have resolved: "This year, I'm really going to lose weight and keep it off." Considering that about 30 percent of our adult population -- and 10 to 20 percent of all teen-agers -- are estimated to be overweight, it's clear that this well-intentioned resolution will be broken almost as soon as it's made.

It need not be. To be sure, loss of weight is never easy, especially if you want to keep it off once it's gone. But if you know and apply a few simple rules, it is possible. Indeed, there is no reason whatsoever why you can't lose weight, enjoy your food, get into better physical shape, and end up with a healthier long-term diet, all at the same time. In this four-part series of articles, we will be telling you how-And we will also offer sample menus, recipes and exercises that not only will help you get started but will keep you going.

First, though, you should know that there are a lot of people who try to reduce who really shouldn't. Pregnant women, for example, should never go on a weight-loss diet. Many teen-agers, girls in particular, judge their appearance not by the way they actually look -- which may be very attractive indeed -- but by how far it deviates from that of the model or film star who's the idol of the moment. Being pencil-thin should not be the goal of most teen-age girls.

Most adults are more realistic about what diets can do for them. Still, many people assume that their "ideal weight" is supposed to be the number given for their height and "body build" in life insurance tables. In reality, these tables are not always applicable to specific individuals. Moreover, they are not based on scientific studies.

The one thing you must always remember is that you can't lose bone, and you don't want to lose muscle, So before beginning our diet, why not check to be certain you are too fat?

Take off all your clothes and look at yourself in a mirror. If you look fat, you probably are. Are there obvious bulges of flesh?

Next, lie flat on your back on the floor and put a yardstick down along a middle line of your body. It should touch both the chest and the pelvic area. If the ruler rides above them, you're carrying too much stomach fat.

For men, there's what we call the belt-line test. The circumference of the chest at the line of the nipples should be greater than that of the abdomen at the line of the navel. If your stomach circumference is larger, you are a prime candidate for a diet.

Finally, there's the "pinch" test. Over half the fat on the body is just under the skin, so this method gives an accurate estimate of the total amount of body fat. To measure yourself, stand erect and let your arms hang free and relaxed. Ask your spouse or a friend to pinch up a fold of skin (not the muscle) at the back of your upper arm, halfway between elbow and shoulder.

The fold should measure between one-quarter and one-half inch thick. If it's much less than half an inch, you're "underfat," no matter how square and solid you may look. If it's more than an inch thick, you are obese or approaching it, and should surely consider beginning a reducing program. And you can expect to be on it for a long time. Go to a doctor to get a sensible diet. Don't just try to do it yourself.

If the fold is between one-quarter and one-half inch, you do have some fat to lose, but not an excessive amount. The question is how much, how fast, and exactly how?

A teen-ager who thinks she (or he) is more than five to 10 pounds overweight should consult a physician: what teen-agers think and what they need are two different things. In the teens, your proportionate need for calories and nutrients is greater than it will ever be again.

Even if you only want to lose two or four pounds it is unwise to try to cut the daily amount of calories below 1,400 to 1,600 for a girl and 1,800 to 2,000 for a boy. If you go below these figures, you can't count on getting all the nutrients you need. Moreover, you will stop any residual growth in height you may be hoping for.

Women over 21 and men over 25 are beyond the growth period. Thus, if you were healthy and felt you looked your best then, your weight at that age is a good level for your personal goal. However, if you decide to lose more than 20 pounds, take the time to consult your doctor first. Above all, don't cut the number of total calories in your daily diet below 1,200 calories for a woman and 1,600 for a man.

Our body puts fat on in tiny increments as a result of eating a few extra calories a day. It's always tempting to go on a crash diet, get rid of it in a hurry, and then go back to normal eating. But please don't. Many of these diets are lacking in some of the essential nutrients. Some, like many socalled "high-protein" diets, which are in reality high-fat, are dangerous to your cardiovascular health. Some, like the liquid protein fast, are dangerous to life itself.

Weight control is not enough. A good diet should help you to establish a pattern of healthy, lower-calorie enjoyable eating -- and a pattern of healthy exercise -- that you can follow for a lifetime.

In our next column, in the Jan. 11 Food section of The Washington Post, we'll show you how to go about it.