Believe it or not, many children express a desire for playing a specific instrument early in their lives. Some of that desire comes from exposure to musical instruments from listening at home, preschool music classes, and concerts that they have attended. If your child starts talking and showing excitement at learning a particular instrument, (i.e. harp, guitar, violin, piano) please believe that specific instrument is probably “the one” for them.
Of course, not all children know what instrument they would care to play so it’s always a good idea in those cases to start with the basics. Piano and singing are very good instruments to learn before branching out to other instruments later.