@Saddam: At the first thing that opens, click "Add" (after choosing your nicknames) and type "irc.relic.net" without the quotes. I was running into trouble connecting after this, but I may've found some sort of fix. Select that and click "Edit". In the window that pops up select whatever's in the selection box and click "Edit" again, then change what says "newserver" to "irc.relic.net". Then click outside of it and see if it works from there.
Once that's done, just go to the chatbox, type "/join #theflatearthsociety" and you should be good to go. You can set it up so it'll join that channel immediately whenever you open HexChat, which you can either figure out yourself or ask me and I'll try to help! I've never used this before so I'm not sure how much I can do if anything goes wrong, but I'll try.