To join a guild you need to have been invited by one. To accept an invitation, click on the tab on the bottom left-hand corner that shows 3 people (next to the building tab). It's the global tab, and if you click it, then click on the shield that comes up, then you'll be taken to a screen where you can view guild invitations or the option to create your own guild. If you are already in a guild, then this will take you to your guild page and forum.