irssi is a great IRC client. The documentation is excellent, but long, so here is my summary of critical usage.

Connect to freenode as nickname:

$ irssi -c -n nickname

Get help:

irc> /help
irc> /help <command>

Join the #phys305 channel:

irc> /join phys305

Leave the current channel:

irc> /part

Quit your IRC session:

irc> /bye

Change to window <N> with Meta-<N>. The Esc key is one of a number of ways to change windows.

Talk by typing whatever you have to say. In a multi-user channel, it is good practice to preface responses with the nick of the person to whom you are responding. This makes it easier for them to pick relevant messages out during parallel conversations. irssi makes this easy by autocompleting nicks with Tab.

You can also look over the beginner section of the IRC mini-HOWTO. If you want to start a new channel, read The New IRC Channel Operator's Guide.