We go to a family dentist that specializes in pediatric dentistry as well, and they do the first visit/cleaning sometime between age three and four (they say 3.5 is ideal).
At the first visit, they had me sit in the chair with my son on my lap. They showed him all the instruments, let him touch everything, told him what everything did, turned on the polisher and has him feel it with his finger, had him practice rinsing and spitting, and sucking the vacuum straw. She had him open his mouth really wide and counted his teeth. Sometimes this is where the first visit ends, but if the child is comfortable and doing fine, they will continue with a cleaning.
My son was doing great, so they went ahead with the cleaning. The first cleaning involved polishing and flossing, but no scraping. When it was over he got a new toothbrush and his photo taken for the "no cavity club" wall.
They usually wait until the second or third visit to do x-rays, unless they feel it is necessary sooner.
Each dental practice likely does things a little differently though, so you may want to call around and ask and find one you are comfortable with. My son loves his dentist, and actually enjoys going. It is wonderful.