You can begin using a bit of toothpaste on a babies teeth once they begin to come on which is around 6 months of age.
However I would wait until the baby is at least 1 year old or a bit older before brushing there teeth because they can't really spit the toothpaste out properly at 6 months of age.
And besides that the babies teeth will eventually fall out when they get to 6 or 7 years old anyway so it's no big deal to not brush the babies teeth when they first come in.
I didn't begin brushing my babies teeth until 1 year of age because by then they could spit the toothpaste out better.
Just begin by using small amounts of toothpaste and brush their teeth and start letting them hold the toothbrush as well.
By 2 or 3 years old the child should be able to brush their own teeth but always supervise them until after 5 years of age with the toothpaste so that they don't swallow it.