When you connect to the irc channel you usually receive a message from the irc server inviting you to register if you are not already registered or to change your nckname if it is already in use.
To be identified by the server you have to type in the chat string the following command:
/msg NickServ identify <password>
If you use AmIRC, after registration you can enter the password in the server selection window.
I am currently connected to the freenode server and use the following parameters:
Server name: irc.freenode.net
Port: 6667
