There are two ways to go at it:
First create your character. Then search around for the guilds that would better suit it. Look around ingame for members of this guild (this way you'll see if the members are active during your active play time) and pose questions. If you like what you see, ask to be recruited, or wait for an invite (if you think you've impressed them enough.) If you don't like what you see, have your character be a freelancer or create a guild yourself.
Second: Look for (or create) a type of guild that thrills you. Create a character that would fit that type. Ask questions ingame and either ask to be invited, wait for an invite or invite people yourself if you have chosen to create a guild. Remember the part about making questions. It's important that your character interacts with other characters before deciding anything guild related. Unless your character is a light minded individual.
It's quite simple, as you see. Use the guild list and the search function to search for guilds.
EDIT: I'm assuming you've read the
player,
roleplay and
guild guides already. If you haven't... Do it now.
EDIT2: My example was the first. It took me two or three months before I joined my first (and current) guild.