The number of young people entering the job market -- now through the 1990s -- is steadily declining, according to the Department of Labor. And that, say Robert and Anne Snelling, translates into job and career opportunities "unparalleled in the history of our nation."

The Snellings, founders of an international employment service headquartered in Sarasota, Fla., predict 20 million new jobs will be created in this country over the next decade: "Instead of individuals competing for the best jobs, companies will be competing for the best people."

To help people who are considering career changes and those entering the job market for the first time, the couple has written Jobs! What They Are . . . Where They Are . . . What They Pay! (1985, Simon & Schuster, 279 pp., $8.95).

According to the Snellings -- and Department of Labor statistics back them up -- the hottest growth will be within the service industries: "Communications, finance, medical care, insurance, high technology."

Three of the top entry-level fields in the Washington area (and nationally as well):

Computers

Computer service technicians -- starting salary $19,500-$23,000. "No matter how great the machines," say the Snellings, "there will always be a need for those who know how to repair and maintain them."

Systems analysts -- starting salary $22,500-$28,500. "Wherever there are computers there will be a need for systems analysts to help utilize them most efficiently."

* Engineering

Starting salaries: $22,000-$30,000.Biomedical engineers -- "use engineering principles to solve medical and health-related problems."

Chemical engineers -- "design equipment and develop processes for ways to use chemicals in research, production and manufacturing."

Electrical engineers -- The military, computer and communications industries represent a continually growing market for electronic equipment.

Petroleum engineers -- "develop and implement new ways to recover and process oil and gas and provide technical consultation during drilling operations."

* Sales

"Across the United States, companies are making an all-out effort to find sales trainees. Those that are hired are moving quickly into management." Entry-level salaries: $15,000 to $30,000 a year. Experienced salespeople can earn from $30,000 to $100,000 and more.

Entry-level possibilities include advertising sales representative, $12,500-$22,000, plus commissions; computer sales rep, $17,500-$24,500; food sales rep, $15,000-$21,400; manufacturers' sales rep, $17,000-$23,600, and insurance sales rep, $16,300-$19,600.

Other high-potential entry-level fields:

* Health services and technology

Employment in this broad field "is expected to grow quickly . . . because of the expanding health care needs of a growing and aging population." Entry-level workers are particularly needed as nurses, $15,600-$18,900; physical therapists, $16,700-$23,300; electrocardiograph technicians, $12,600-$14,800; emergency medical technicians, $12,000-$17,000, and X-ray technologists, $15,000-$17,500.

* Finance

"The problems of inflation combined with deregulation of the banking industry have brought vast changes to the world of finance . . . All these changes mean great opportunities in the financial field. Five good entry-level opportunities: Junior analysts, $18,000-$22,500; stockbrokers, $14,500-$25,000; banking trainees, $18,600-$21,400; claims representatives, $16,200-$17,600 and accountants, $17,500-$22,500.

Another job-guide source: The Bureau of Labor Statistics' comprehensive Occupational Outlook Handbook (Government Printing Office, $8.50).