You'll need an XMPP client to communicate on the XMPP network. We recommend Gajim (no calling functionality) and Kaidan/Dino (with calling functionality) for Linux/Windows PCs.
For Android, have a look at Conversations, which costs money on Google Play (but not on F-Droid).
Different web versions exist, we host converse.js, which is locked down to only our domain (public instances exist!). Be aware that our instance does NOT support OMEMO, and should not be used for private conversation. Movim is another interesting one, which has a large social media aspect to it as well, though we do not host it.
More alternatives are available at XMPP.org's software page.
To join, you will be able to register through whichever client you want. Someone who is already registered will need to vouch for you, though, or your account will be deleted rather quickly.
You can choose a username (it will end in "@fridyrk.eu", just like email!), and a password. You're on the XMPP network now! Tell your friends your username, add each other as contacts and whatnot. Don't forget that you can connect across XMPP servers.
Your client SHOULD support OMEMO encryption. If it doesn't, we'd advice you switch to one that does. Instructions are different per client, but may be available as a lock button somewhere in a chat. Make sure that it is on/set to OMEMO. No-one will be able to read your messages, not even us!