How to breed with a female starter in Pokemon X?

I started with a female Froakie in Pokemon X, but I did not know you had to have a male to breed. I finished the game but I want to breed my Greninja. How can I do that?

You can breed almost any Pokemon with Dittos regardless of gender. Ditto can be caught in the Pokemon Village or in certain Normal-type Friend Safaris.

Alternatively, as your Greninja is female, you can breed it with a male of a different type of Pokemon, so long as the male is in the same egg group as the Froakie evolutionary line.

Froakie and its evolutions are in the Water 1 egg group, which contains 108 Pokemon. For example, Poliwag, Psyduck and Marrill are all in Water 1 and relatively accessible to catch in-game.