How do you catch Charmander in black and white?

You can't catch Charmander in Pokémon Black and White. Charmander is a Fire-type Pokémon that is exclusive to the Kanto region, which is not featured in Black and White.

You can, however, get a Charmeleon via the Dream World, but it's a rare occurrence.

If you're looking for a Fire-type Pokémon in Black and White, you can find Tepig as a starter, and other Fire-types like Pansear, Vulpix, and Flareon in the wild.