NEXT SATURDAY afternoon William Fred Scott III will make his debut at 26 with the New York City Opera Company, conducting the first of three performances of "The Barber of Seville" scheduled for the company's spring season.

How does a young man who grew up in Thomasville, Ga., graduated from Georgetown University's School Foreign Service, and occasionally wrote music reviews for The Washington Post, land in so desirable a spot? (Scott is of course not the first man from a small south Georgia town to make his mark in Washington and New York.)

You could say that Scott arrived at his new eminence by following some advice Leonard Bernstein has often given young musicians (even though Scott has not yet met Bernstein, and did not know about the advice). When the young violinist James Oliver Buswell IV, having won major violin competitions, was wondering what to do about a college career, Bernstein urged him to go to Harvard. "Not," said Berstein, "a conservatory. Go to Harvard [Bernstein's alma mater] and get a academic degree. You can play the violin as much as you want."

Scott's road to the New York City Opera was not via the competition route. He came to Georgetown's Foregin Service School because his father thought that would be a logical and useful place for a young man with a flair for languages and a smooth, polished manner. But Scott, who had already become an outstanding pianist in Thomasville (at the age of 12 he felt that the Grieg Concedrto "belonged" to him) and had also mastered the organ in handsom fashion, was interested in Georgetown because Georgetown was in Wahsington and there was a lot of music in Washington.

So, arriving in Washington in the fall of 1970, Scott soon found himself studying music at Georgetown and spending every spare moment at concerts, operas, Washington Cathedral, and any other place where he could hear music. By the time he was a junior, it was obvious to him, his advisers and friends that the only career for him was in music. Continuing along what is usually described as the classical European route by which conductors achieve experience and a solid basis for growth, Scott served not only as pianist and organist but also as an assistance choral conductor.

His first big opening came in the summer of 1974, shortly after his graduation from Georgetown, where he had polished up his French and Spanish and rubbed shoulders with German. Sarah Caldwell came to Wolf Trap to stage her production of Prokofiev's War and Peace." One day she was conducting piano rehearsals for which three pianists were alternating. Suddenly she announced, "I want that one and no one else." That one was Scott.

The following January, Scott went to Boston to work for Caldwell and the Opera Company of Boston, where he now performs the dual function of associate conductor and artistic administrator. One Saturday morning in June of '75, Scott had a phone call. Sarah Caldwell had been taken to the hospital and Scott was to conduct that evening's performance of Bellini's "I Capuleti ed I Montecchi," by Bellini, an work not often performed, and not in the repertoire even of seasoned conductors. Scott had been involved in some of the musical preparation of the work, knew the singers and the orchestra. But never before had he conducted any full-length opera, and there would be no rehearsal.

Beverly Sills and Tatiana Troyanos headed the cast, fast company for a 22-year-old newcomer. Instead of wishing him luck or urging him to break a leg, Sills smiled at him before the curtain and said, "Remember, I'm the big girl with the red hair." The performance that night went so well that the newspapers, which had already covered the opera's opening, reported that Scott was at ease, had the music well in hand, and added touches of his own. Three days later Sills called a friend of the young conductor and said, "I want to tell you that Scott is something!"

Now that Sills is running the New York City Company, and in a position to hire whom she wants, it's clear she meant what she said about Scott. He will conduct not only this spring's three Barbers, but also will conduct it when Sills sings it in Philadelphia's Robin Hood Dell in June, and will again be in charge of it in the opening week of the fall season, after which he will go with the company to Los Angeles.

Meanwhile, Scott continues his work in Boston, where he has now conducted well over 100 performances for Opera New England (the traveling road company operated by Caldwell and OCB) as well as in the company's new Opera House, formerly known as the Savoy Theater. It was Scott who inaugurated the new house in November 1978 when he conducted "Tosca" with the legendary Magda Olivero in the title role.

Caldwell has also sent Scott on other European errands and had him accompany her on the investigating trips in which she looks into archives and other sources that lead to the kind of excitement for which she has made her Boston company famous. The latest of these was her study of one of the world's most popular operas, "Madama Butterfly," of which Caldwell gave the first U.S. performances in the composer's original two-act format with a number of passages that are quite unknown to the average "Butterfly" audience. This will be heard at Wolf Trap on June 13 during the visit of the Opera Company of Boston, one of whose performances Scott is scheduled to conduct.

The young pianist-organist-conductor has also served in other capacities. When Sarah Caldwell persuaded Jon Vickers to take on the title role in Berlioz' "Benvenuto Cellini," the noted Canadian tenor, who has often sung for Caldwell, said, "I'll do it for you, Sarah, but you know what's going to happen. I will do all that work learning it, sing it for you three times and that will be the end of that. So -- you send down a topnotch coach to teach me the music. I want someone who will make me work!" Sarah sent Scott, who came back filled with admiration for the intensity with which Vickers works on any new project. But Scott was honest about adding that going to Bermuda to spend a week with the delightful Vickers family, swimming and sunning, was not too hard to take. When the Cellini had been learned, Vickers told Scott that if he decided to sing Tannhauser (a role he finally refused because he said he simply could not get "inside" it,) he wanted Scott again as coach. He also added that he sang few song recitals, but that when he did Scott should be his pianist.

Every young musician who has reached some of the higher steps on the ladder that leads to the top wonders what he will do to continue his growth and development. Scott has been approached by Robert Shaw about the possibility of joining the conducting staff of the Atlanta Symphony some time in the not-too-distant future. He frequently accompanies Caldwell when she is the not-too-distance future. He frequently accompanies Caldwell when she is out of the orchestral circuit, where she has been increasingly busy in recent years.She said not long ago, "I go through every score that I conduct with him, and he knows them thoroughly." At this point he has handled the musical preparation for close to 50 operas for the Boston Company. With his opening in New York this week, William Fred Scott III will be watched by more than the people in the audience in the State Theater.