Make sure you've got the necessities
To create a guild, you first need to gather a full group of players (five members plus the new guild leader.) The guild leader must be at least 15th level and have 10,000 credits (which will be removed when the guild is created.)

Before you create your guild, you'll also need to think of a good name for it. Once you've decided upon a name, have a full group, are 15th level, and have 10,000 credits to spare, then you're ready to form your guild!

How to Create the Guild

To actually create the guild, open your chat interface and type /gc create "your guilds name" (without the quotation marks). There, you've formed your guild! Now, look over the list of guild commands to see about promoting members, recruiting new members, and all your other options!

You can also create a guild by clicking the Group button on top of your chat interface, then choosing Guild Info.