To get your kid to brush their teeth you should try to make it fun.
Also consider using a teddy bear and show them that it can be fun to brush their teeth and let the kid start by brushing the Teddy Bears teeth even though the teddy bear has no teeth.
My dad did this to my 4 year old son to get him to brush his teeth and he started brushing his teeth and also loved pretend brushing the bears pretend teeth.
Also try brushing your own teeth while they are watching and let them know that brushing their teeth is a good thing.
Maybe show them some pictures of what can happen if they don't brush their teeth.
Sometimes you need to brush the kids teeth yourself until they do it on their own and you should help the kid brush their teeth until at least 5 years old and supervise the kid until 7 or 8 years old to ensure they brush their teeth.
Maybe try brushing your teeth and dance to some music the child also likes and the child will usually want to go along with you and do what you're doing.
Also check out some tooth brushing videos on Youtube and there are some fun tooth brushing videos on Youtube such as with Blippi.
Blippi has a good tooth brushing video that my kids like and they also like Blippi as well.