Let them watch you brush. Take them to buy a toothbrush, perhaps a musical one or one with a handle that changes colour. I like Safeway's Oracle milk teeth for its long handle and short head. Superdrug stock a musical brush. Let them put it in their mouth and chew on it even before they have teeth - so they get used to having a brush in their mouth.
Many children dislike peppermint, possibly because peppermint can aggravate reflux.
Make a game of it - let them clean your teeth while you do theirs, sing a song, look in a mirror, pretend their mouth is a tunnel and the brush is a train or a hangar with an aeroplane. Some parents have found health visitors helpful in showing them how to hold a child from behind and tilt the head back. Let them sit on your lap. Suggest that when you clean their teeth it cleans teddy's too and teddy wants shiny teeth. Say I bet I can open my mouth wider than you can.