How do I encourage my 15m child to brush her teeth?

You can bring her along when you buy her toothbrush so that she has the decision to choose which toothbrush she wants to buy. Toothbrush nowadays comes in different cartoon designs so it might spark some interest in her. Also, by allowing her to choose her own toothbrush, you can help generate like a "buzz and excitement" by talking to her about it on the way home e.g. asking if she's exciting to use her new Disney Princess toothbrush so that her teeth will be white like the Princess? This method works for my daughter but that also meant that once a new Disney cartoon comes out, there's a chance that she'd want a new toothbrush lol