How do you get married in fire emblem fates?

Marriage happens as soon as you reach S-rank Support with someone.

As a result, even though every character can have as many C, B, or A Supports as you wish, they can only have a single S-rank Support.

Building Support is fairly easy, though it takes time:

When a character attacks or gets attacked by an enemy while paired up with or adjacent to a compatible ally, that character receives points towards the next Support level with all those allies.

After the fight concludes, you'll see a heart symbol appear above the character and all allies they've built Support with.

Using the Seed of Trust also helps building Support. Simply have your characters pair up, and use the Seed.

The number of points you need for the next Support rank depends on the character and the Support level you're working towards.

Keep in mind that only your avatar can have Support ranks with all other characters. To see which characters can Support which, check the "Support" option during Battle Preparation.