You can download the discord app for various operating systems here: https://discord.com/download
On Linux, you can also install the discord app via snap:
sudo snap install discord
If don't have an account already, click on "register",
you will need a valid e-mail address.
Once you're registered, log in with your username and password.
If you also have the discord app on your phone, you can also scan the QR-code to log in on your computer (e.g. option "Scan QR Code" under "User Settings" in the Android app.)
You will receive an invitation to the course server via e-mail or rocketchat