By the age of two and a half, the child should have all 20 deciduous or baby teeth and they will need to be checked by the dentist. However, it really depends on the individual child's temperament. If the child at that age is still very fidgety and cannot sit still for a couple of minutes, then it might be more trouble that it's worth to get them in the dentist's chair.
The best way to help your child is to get one of those children's picture books (like Berenstein Bears) that depict a visit to the dentist. Read through the book a few times with the child in the days and weeks before the appointment. Point out the chair, the light, the dentist, the assistant, the mouth mirror in the pictures. Make the trip to the dentist something exciting for the child, even a treat. Then, if an older sibling is already a veteran of the dentist's office, get him or her to model excellent behaviour in the chair for the younger child before putting the child in the chair for his or her turn. Sometimes, it may be necessary for you to sit in the chair and hold the child in your lap for the first examination. As long as you have a nice dentist who likes treating kids, it should be fine.