Ideally, you’d start cleaning the baby’s gums before the first tooth even pops out. You can get a little finger brush to use with tooth and gum cleaner for babies—no flouride until the child is old enough to remember to spit it out instead of swallow it.
Also, our dentist said that kids should have help brusing their teeth until they are 8 years old because until then their manual dexterity isn’t developed enough to do a proper job of brushing the teeth. With older toddlers and up through preschool, I let them have a turn brushing first in the morning. Then I take the toothbrush just to make sure all the teeth get cleaned. At night, I get the first turn, so I know they are going to bed with clean teeth. Most of the time I help them floss at night too (but sometimes we skip that, just don’t tell our dentist!).