All players join guilds automatically at level 10. Your guild will be chosen on a casual basis from those that were already created by the server.

You can join a guild by submitting a request and getting approved by the guild leader, or simply by accepting an invitation.
Find a private guild by searching its name, then hit the Request button. Once the guild's leader approves your request, you'll automatically join the guild. All your requests to other guilds will be automatically withdrawn.
You can send out a maximum of 10 requests. If you send out more, the older requests will be automatically withdrawn. If a guild leader does not respond to your request in a month's time, it'll be deleted.
In the same window you can find a particular guild by name, type, or minimum level.