It's very important to manage your child's and to teach them how so they learn to do this themselves. As a baby and with teeth starting to appear, at around 6 months old, just a pea sized amount of baby toothpaste on your little finger is enough. As teeth grow you can progress to a softer baby toothbrush and brush along their teeth with them.
Use a flouride toothpaste to prevent decay and make sure your child gets used to a brushing routine in the morning and before bed. You can introduce a sticker chart to encourage good teeth hygiene. A sticker or a nice big toothy smiley face (they can draw this themselves) can encourage good habits.