If you find yourself stuck behind a firewall (or similar) and are unable to reach the usual IRC ports (6667 and 6668 for regular access; 7000 for ssl), you can try some of the alternate ones below:
|caladan.shadowfire.org||21, 25, 53, 110, 443|
If you're new to IRC, you'll first need an IRC client before you can start chatting. Wikipedia has an extensive list of IRC clients for all platforms.
If you're unable to install new software or just unable to use one of the above IRC clients, then Mibbit might be helpful.