It doesn't matter if your child is a toddler or about to graduate high school -- almost every young person has some form of fear of the dentist. Whatever it may be, getting the child to the dentist is often the easy part -- actually being treated by a dentist is an entirely different level of challenge.
One of the best things you can do to improve your child's relationship with the dentist is to establish your child's relationship with the dentist. That means taking them in regularly -- at least twice a year -- when they're quite young. Three-year-olds are starting to form long term ideas about the world around them, and learning about the dentist at that age is entirely appropriate even if it it's just to bring them in for a five minute checkup and to become familiar with the dental office and the surroundings.
Another relatively simple step you can take is to arrange to be in the dentist's office with them. Up until about the age when they don't want to be seen getting out of your car at school, they'll be perfectly happy to have you come into the dentist's office and sit next to them while they get their teeth cleaned -- and there's nothing wrong with that.
You can also help by explaining to them exactly what the dentist is doing. It's one thing to say "he keeps your teeth clean" -- and that works quite well for single-digit kids -- but once a child gets into the tweens and teens, they deserve more. Talk to them about oral health in specifics. Ask the dentist to show them some dentures and talk about how to avoid ever needing them. The more they understand how the dentist is helping their future selves keep eating, the more they'll see the dentist as an ally rather than someone to be feared.