To join a guild, it depend on which faction you've selected before you can join a guild at level 20.
If you selected Primus Union, you can join guild that is on Primus side only. And same goes for Ganav Libero.

In order to join my guild "Angelwing", you got to be Primus Union.
So how or where to choose faction?
First, you got to get yourself trained till level 20.
Go to Lunatia, accept the quest call "The Road I Have To Go" from the NPC: "Rudy Hoshake" which located in the map below.

Follow by the quests below.
"Important News! Primus Union" from NPC: Roopius Xarapa see map above.

Then follow by "Important News! Ganav Libero" from NPC: "Rilly Shopat see map above.

Next is "Time of Decision" from "Rudy Hoshake" again.

Last quest is "Jeff, Emissary of Primus Union" from NPC "Jeff". See map below.

Now you're a Primus Union ! Congratulations!

Next, PM me to join guild!