When you make your character, which has to be done before jobs can be started, you'll have to list a guild. If you're unsure as to which guild you wanna join you can input "Guildless" on the app. Once you figure out which one is for you a mod can re-open the application and you can change it.
I normally ask players who wanna join a legal guild and don't really care which to join LS. We're a small guild due to unfortunate circumstances and certain members of the community enjoy belittling use because of that, but our community is relaxed both in and out of character, and if you're worried about being weird; the guild currently contains: An angel, a half dragon, a guy who walks with spirits and voices in his head, a human, a midget, and a chick who's half demon/half angel... there's always room for more weird people xD
For those who would rather a guild that fits with their character's profile, I'm more than happy to talk with them about which guilds can help them out.