The short answer is…before they get teeth, around 5 months. Start by wiping the gums with a clean cloth or rubbery finger cap. And schedule a dentist appointment after her first tooth appears, at about a year old.
This youtube shows how to brush teeth in the littlest ones
Start to brush their teeth with a toothbrush at around a year old. Learn from our mistakes. Our daughter had 7 cavities at age 4, her first dentist appointment! That was too late!
Make brushing fun and kids will do it more readily. Starting the habit young will help a lot. Good luck. I know you’ll do better than we did.