Don't be afraid - it's only a machine; Electronic Life - How to Think About Computers, by Michael Crichton. New York: Alfred A. Knopf. 209 pp. $12.95.

If you want to be the only grown-up kid on your block still living in the horse-and-buggy age, don't learn to use a home computer, advises author Michael Crichton.

If Crichton's tone is tongue in cheek, his guidelines for buying and using a computer are practical: Plunge in and buy beginners' equipment; steep yourself in computer lingo; buy a monitor with an on/off switch so you won't have to unplug your mechanical monster each time you finish using it. Keep a notebook of your mistakes, and don't be afraid. After all, Crichton admonishes, ''It's only a machine.''

If you study the ''how-to'' primer and still think you don't need a computer, you may be right, he says. Just remember: ''You probably don't need a TV either, and you almost certainly have one.''