The standard IRC client is the original ircII client. It's part of most Linux distributions.
It's easy to use ircII. Let's say you want to connect to irc.openprojects.net as mini-HOWTO.
At the command line, type:
$ irc mini-HOWTO irc.openprojects.net
You can also export variables, so you won't need to use them at the command line:
$ export IRCNICK=mini-HOWTO IRCSERVER=irc.openprojects.net
Add them to your shell profile (e.g. ~/.bash_profile or ~/.zprofile) when you're done.
Other common variables are IRCNAME and IRCUSER, to respectively set the ircname part of a /whois and username as seen at the first line 'mini-HOWTO is ~username@hostname (ircname)'. Keep in mind that IRCUSER won't work if you run an ident daemon (default on most distributions). If you still need to change your username (not recommended, and I hope you're not using IRC logged as root !), install oidentd from http://ojnk.sourceforge.net/, create /etc/identd.spoof with a list of users allowed to spoof their ident, and ~/.ispoof with their reply (e.g. reply). Finally run '/usr/local/sbin/oidentd -g nobody -N -s -u nobody'. Add this to your startup scripts (e.g. /etc/rc.d/rc.local) when you're done.
If not set, IRCNICK, IRCUSER, and IRCNAME will be retrieved from /etc/passwd .
Use /help to get a list on all available commands (/help help is a good start). Replace nick by any IRCNICK.
First, /set NOVICE off
/nick IRC-mini-HOWTO changes your IRCNICK to IRC-mini-HOWTO
/set realname The Linux IRC mini-HOWTO changes your IRCNAME to The Linux IRC mini-HOWTO (doesn't change on the same connection)
/j #LinPeople joins channel #LinPeople
/j #Slackware joins channel #Slackware
/j #LinPeople changes the active current channel to #LinPeople
/msg nick Hi. sends a private message to nick containing Hi.
/notice nick (or #LinPeople) Hi. sends a notice to nick (or #LinPeople) containing Hi.
/query nick starts a private conversation with nick. /query ends the private conversation
/me loves Linux. sends an action to the current channel or query containing IRC-mini-HOWTO loves Linux.
/dcc chat nick starts a chat with nick. Use /msg =nick (notice the =) to send messages over the chat
/dcc send nick /etc/HOSTNAME sends the given file to nick
/dcc get nick receives the file offered by nick
/part leaves the active current channel
/part #Slackware leaves channel #Slackware
/discon disconnects from current IRCSERVER
/server irc.dal.net connects to IRCSERVER irc.dal.net
/quit Bye. quits your IRC session with a reason Bye.
WARNING WARNING WARNING WARNING WARNING
Never use IRC logged as root or any user with excessive privileges. Bad things may happen sooner or later. You were warned. It's safe if you create 2 users, one of them to only use IRC.
$ man adduser
On Linux channels you shouldn't:
Act as an idiot. If you want to be respected, then first respect each other.
Use colors (^C). Most Linux users don't tolerate such mIRC crazes, and ircII doesn't really support them. The same should apply for ANSI.
Use full CAPS, bold (^B), reverse (^V), underline (^_), blink (^F), and bell (^G). The first 4 are here to emphasize words, not the whole text. The last 2 are just very annoying.
Ask if you can ask a question. Just ask, but first read all documentation available on the subject. Start looking at /usr/doc/ , otherwise go to http://www.linuxdoc.org/ or http://www.ibiblio.org/pub/Linux/docs/. And don't repeat your question immediately. Wait at least 10 minutes. If you don't get any answer it's because nobody knows or wants to help. Respect their choice, they're not your personal assistant. Also never send mass private messages. It's like SPAM.