Well, unless you tell us what client you installed it will be quite hard to tell you how you have to start it. If you want something graphical i'd recommend irc/hexchat. If you prefer CLI i think irc/irssi is popular. If you go for irc/hexchat there should be a popup the first time you run it (i figure it would be in your application menu or something? - sorry, i have no clue about MATE) where you can select a network. Choose freenode. With irc/irssi i think
irssi -c irc.freenode.net -n yournameshould get you connected.
Once your on the server you should register your nickname (see https://freenode.net/kb/answer/registration for instructions). Once that's done you can join the relevant channels with
/join #freebsd-social. You can also join without registering but only the #freebsd-social one as the other requires registration. Good luck
Thank You! I'm now registered and looking forward to re-establishing with users of FreeBSD!
Nick Name: Rp47