XChat is tons better, I use it (even on a Windows platform) <!-- s:P --><img src="{SMILIES_PATH}/icon_razz.gif" alt=":P" title="Razz" /><!-- s:P -->
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Quite easy to use and it's quite stable too. Plus it's free software and not nagging shareware like mIRC is.
I do like Mozilla's apps and stuff, but Chatzilla I don't like as it's extremely basic.
The command to join a channel is /join, for examples:
/join #gargoyle
/join #korax
As for getting an account registered and authed...
/authserv register nick password email
DO NOT forget the forward slash, and do this in your server status tab! Otherwise you might risk getting this posted in the irc channel for others to see D:
After that, check your email for a "cookie" to authorize yourself with. Copy the command and enter it in your server status tab. (Instead of the channel tab)
THEN YOU'RE DONE!
Actually, almost.
To log in, type this in your server status tab
/authserv identify nick pass
AND WOW YOU'RE AUTHED!
You can also add this into your perform settings (mIRC) or your Connect Commands (Xchat, but without the slash)