So you've decided to cut back on the beer, fried chicken and twice-baked potatoes (with cheese, please!) and make an earnest effort to reshape the old bod. But you're not sure if you want to invest in a wardrobe of nifty neon workout wear and sign a year's contract at the neighborhood sweat emporium. A good beginning would be a brisk walk to the nearest bookstore to browse through the physical-fitness shelves (next to the cookbooks, right?). Choose the format that you think will work. Here are a few of the "starting blocks." On your mark . . . Working Out "Jazzercise" by Judi Missett (Bantam: $3.50) approaches aerobics from a novel angle--exercises are basic jazz-dance movements. The twisting, stretching and reaching motions are limbering-up exercises that lead to more demanding splits, kicks and leaps. Whew! The medical side of aerobics is discussed by Kenneth H. Cooper MD in "The New Aerobics," "The Aerobics Program for Total Well-Being" and (co-authored by his wife Mildred) "Aerobics for Women" (Bantam Books: $3.95, $10.95 and $3.95). The books include well-balanced diets and statistics, charts, lists and equations that document various exercises. If you like to "keep score," there is a point system that serves as an incentive. "Gravity Boots: Hanging Your Way to Health" by Tanya Slover (PocketBooks: $5.95) explores one approach to back and circulatory care. One should study the pros and cons of gravity inversion before embarking on this form of body improvement, and a reading of the book's disclaimer and warnings is definitely called for. "Staying Fit!" by John Travolta (Simon & Schuster: $15.95) is a crash workout program for exercising six days a week, the way the actor-dancer prepared for his leaps and bounds in the movie "Staying Alive . " Combining dance techniques for improved aerobic capacity with weight-resistance training, Travolta boldly predicts an incredible new body! Lights, camera--oh, forget it . . . . See Dick run. See Dick run fast. See Dick reading "The Competitive Runner's Training Book" by Bill Dellinger and Bill Freeman (Collier Books/Macmillan: $6.95). Dick is learning all about serious running--training for strength, flexibility, tempo, progression, mental attitude and diet. See Dick win. For those who hold no affection for love-handles, these three slim handbooks are concerned with specific body parts: "30 Days to a Flatter Stomach for Men" by Roy Matthews and Nancy Burstein (Bantam Books: $2.95), "30 Days to a Flatter Stomach for Women" by Nancy Burstein (Bantam Books: $3.50) and "Slimming Your Hips and Thighs" by Ann Dugan (Pocket Books: $2.95). A little daily effort can make the difference when you're just a little bulge over your preferred size. "Heavyhands" by Leonard Schwartz MD (Warner Books / Little, Brown: $8.95) explores the use of hand-held weights as a primary exercise or as a complement to assorted general exercises. The author claims hand weights double the effectiveness of all exercises. In 30 days, boast the cover lines, you, too, can look like a chorus girl. "The Broadway Workout" by Shelly Rann (Dell: $3.75) promises a streamlined body that's ready for "The Great White Weigh." The program is divided into three parts: stretch and trim warm-ups, serious-business shape-ups and aerobic routines. And it looks like fun if you're attuned to rollicking stage music and lots of jazzy movements. Ready, Busby? Now that you have stretched your muscles beyond human endurance, there's a mind that needs training. This seems to be the main strength of "Breakthrough: Maximum Sports Training" by Mike Spino (Pocket Books: $5.95). Mental factors, including the positive and negative aspects of concentration, calmness, stress, imagery and emotions, are strongly emphasized in this paperback. Spino presents small scenarios of possible situations that will psych the athlete into an improved performance. Tote that barge! Lift that bale . . . "Lisa Lyon's Body Magic" (Bantam Books / Macmillan: $10.99) expounds on body-building as the fastest and easiest way to trim, slim and beautify the female form. The benefits, according to Lyon, include better circulation, increased flexibility and agility, better-functioning organs, more elastic skin, a reduced fat layer between skin and muscles, fewer poisons in the system, more sensitivity and healthier hair and nails. And all this just by imitating Arnold Schwarzenegger?