Hi, welcome to FFXIV (and Happy New Year)!

There is no main chat channel in the game. There can be some general zone chat in the current endgame hub and city (Rhalgar's Reach and Kugane for Stormblood), but for the most part people stick to FC chat, linkshells or the Novice Network.

As a new player you can gain access to the Novice Network which is a chat channel for newbies and mentors. Just ask any player with a crown above their head to invite you. You will have access until you loose or remove your new player green leaf icon. After that if you want to rejoin you will have to become a mentor.

Linkshells are private, invite-only, player run chat channels. You can be a member of eight linkshells at a time, one for which can be a cross world linkshell for taking to other people on your data center. Linkshells will normally have a theme to them like "hunts" (for the ingame timed monster hunts), raiding, crafting or just general social chat. Players can also have one Free Company (guild) at a time.

Hope this helps!