Why Join a Guild?
The why is easy. This is a social game and the tabs you have are very limited for chat. To get an instant group of friends and your own channel to chat in.
Choosing and Joining a Guild
The best way to choose an active and friendly guild is getting to know people in the Help or main chat windows. Those in guilds will have the guild tag in square brackets directly before their name ( [Time] [Guildtag] PlayersName: ). You can ask people directly in Help or via Whispers/Emails about their guild and do they recommened it.
An active guild with a leader who organizes events and plays regularly is a good sign as part of the benefit of the guild is having plenty of other players who chat in the Guild Tab and want to help.
To Join a Guild
When you find one you like contact the Guild Leader and ask if you would be allowed to join in a very polite email. You can ask a member for the Guild Leaders name or you can easily look it up in the Guild list.
Click the Guilds Button on the Bottom Bar and look through the list of guilds available. Find the one you want and click it. Look for the Guild Leaders name and take note of it. Now email that user name with a very polite email.
New guilds are easier to join as starting a guild gives you a number of free places for members, as they grow they need to buy licences to expand their guild, these cost game cash and you might be asked to make a donation of Gold Coins to be accepted.
Some Guilds have certain restriction about the players level or any number of other possible reasons to accept or deny entry and it's handy to find that out first by asking existing members.
Creating a Guild
If you wish to create your own guild, firstly, you must build a Guild House. You can either buy one in the Architect tab of the Merchant Chest for 1,250 Gems, or find in at the bottom of the Intermediate Buildings tab in your build menu.
Once you have a guild house, you should see a 'Create Guild' button on the guild menu. A guild can have up to 15 members, but if you wish to have more, you will need to buy Licenses, available in the Black Market tab of the Merchant Chest. The maximum capacity of any guild is 100 members.