First you need to create a Discord account you can head to this link to create one here[discord.com]. *PS: you must be 13+ to create a Discord account.

You can find regiments in the official discord server by heading to the #recruitment channel here's the invite: click me[discord.gg].
You can also head in-game main menu and click regiment registry and look for regiments that are recruiting like *cough* 8thlr.

To join the regiment you can join their discord server posted in #recruitment channel or if you are joining via in-game you can click view regiment and click the discord logo. They should help you set things up in their discord server.