My first child went when the first tooth came out. Actually she got her appointment before it came out. They just booked her at the same time as me. They do not do much just feel around their mouth and so they get used to it. They also talk to you about hygiene.
My boy had his first dentist appointment a few weeks before he turned 1. It can be quite hard to get registered to an NHS dentist at the moment so worth having a look in advance to see who is accepting new patients. They like to see them every 6 months.
I was told to go in when they first get a few teeth (around 6-8 months) and then every year from then.