For many children, September will mean the beginning of music lessons. For parents, now is a good time to think about helping their child get off to a satisfying start.

The proper selection of an instrument, says John Stephens, music specialist for Montgomery County Schools, is "almost more important than natural aptitude" in determining whether a student will have a successful musical experience.

It's not easy to make the right decision. Even concert pianist Andre Watts started out on the wrong instrument -- for him. At 6, he took up the violin; a half-year later he gave up and switched to the piano with which he felt more "comfortable."

Lynn McLain, conductor of the D.C. Youth Orchestra, believes strongly that the way in which a child relates to an instrument should be the determining factor in selection. Like Watts, McLain maintains the child "should feel comfortable with it."

"We generally begin instrumental music instruction in the 4th grade," says Stephens, "because by this age most students are physically able to play an instrument. They have the strength and the fine-muscle control needed. Their fingers are long enough to reach the strings or keys. We look at their teeth, too. A child whose permanent teeth are not all the way in, or who is wearing braces, is obviously not a candidate for the woodwind or brass section."

Says James Holloway, a doctoral student in piano at the University of Maryland and also a piano instructor : "By the time a child reaches the third grade, he or she has learned how to read and how to follow instructions." The pupil "has become accustomed to assignments, to completing work, to teacher-student relationships.

"Some children can begin at earlier ages and be quite successful, especially if they are unusually bright, good at recognizing symbols, come from a musically active home and are physically able."

The usual instruments for very young children are the piano -- it's physically easier to produce sound on the piano than on some other instruments -- and string instruments, which come in sizes to fit even small children.

Another consideration is the child's sense of pitch. Stephens gives a "pitch test" at the beginning of each school year. The test, which he devised, measures how well a child can distinguish sounds played on different instruments.

Those who are found to have an excellent perception of pitch will be encouraged to take up a string instrument, or perhaps the French horn. A "good ear" is essential to producing accurate sounds with these instruments.

Other students whose sense of pitch is not so well developed will be urged to consider instruments in which the production of sound is "more mechanical" and less dependent upon the ear, such as woodwinds.

Stephens' test also gives students a chance to hear and see all the instruments of the orchestra.

"Based on all these factors, we try to help the child make an intelligent selection of instrument," he says. "This doesn't mean that he can't change his mind later. Pitch can be learned. The ear can be trained. A child's interest and physical ability may change. What we try to do is help each child select an instrument with which he will have some initial success."

Some Washington-area schools supply instruments and some do not. In the D.C. Public Schools, elementary students can use them free of charge. Prince George's County provides some of the more expensive instruments such as cellos, violas and saxophones. Arlington and Fairfax counties both rent some instruments to students at a cost of $25 to $50 per school year. Montgomery County supplies no instruments.

Purchased new, most student-quality instruments will cost between $250 and $500. French horns and cellos are more expensive. Other options are to buy a used instrument or to participate in a rental-purchase plan. A large portion of the rental fee of $10 to $20 per month will be applied to the purchase price of the instrument.

New pianos are priced at $2,500 and up and can be rented for about $35 per month. Rental-purchase plans are available. Stephens suggests group lessons as a start. Playing alongside others (the way most instruments are meant to be played), children learn by seeing the mistakes -- and good points -- of others, and they get to perform regularly. Group lessons also are less expensive, about $2 to $3 a session compared to $7 to $15 and up for private instruction.

For group lessons to be effective, Stephens stresses, it is important the teacher encourages both the giving and receiving of constructive criticism.

Group music lessons on some instruments are available through city and suburban recreation departments. Some music stores offer instruction in both group and private lessons. Parent-teacher associations and other groups sponsor after-school programs.

The D.C. Youth Orchestra, headquartered at Calvin Coolidge High School, offers free instruction at six different levels from beginner on up, ages 7 to 19. Each year says conductor McLain, 800 to 1,000 children from the metropolitan area participate in its classes, orchestras and woodwind groups.

A disadvantage of group instruction, of course, is that the pupil does not receive as much individual attention. Also, some children may perform better in a private situation, in which they can develop a personal relationship with the teacher. Some parents supplement group lessons with private.

The selection of a teacher, says Stephens, is of no less importance than selection of the instrument.

"You must have a good teacher. One poor teacher at an early stage can cause great frustration. It can take years to undo the damage."

Fortunately, he adds, "One thing we have no shortage of in this area is good teachers."

Many members of the National Symphony Orchestra, the Kennedy Center orchestras, military bands and professional groups teach on the side. Check these sources, ask the school music teacher or call the music departments of area colleges and universities.

"The best of all possible ways to judge the quality of a teacher," says Stephens, "is to listen to the teacher's students, either in recital or in competition.

"A good teacher develops a serious interest on the part of a student in the task of learning to play, and provides a standard of excellence for a student to emulate. He provides some incentives that go beyond the lesson and carry through the week. He shows a student how to practice."

The competent teacher, to Holloway, "omits no facet of a basic music education." This means helping the student with technical development, expressiveness, reading music and theory and giving the student a chance to perform regularly.

For Andre Watts, an effective instructor, above all, helps a student "pick up on the fun of playing.

"There is no other way but to practice, but music should be joyous, fun, pleasant. If your heart's not in it, it's no good."