Each person on an IRC network identifies him- or herself with a unique nickname.
Changing your nickname goes by typing:
/nick <new nickname>
Example:
/nick Kermit- You're now known as Kermit-
The server will give you a message if you select a nickname which is already in use or if you choose a nickname which contains an unacceptable character.
Valid characters for nicknames include: a to z, A to Z, 0 to 9 and the following special characters: back slash (\), backstroke (`), caret (^), dash (-), pipe (|), underscore (_), left square bracket ([), right square bracket (]), left curly bracket ({) and right curly bracket (}).
Depending on the server/network you are on, nicknames might be limited to a number of characters. On AmberNet, nicknames can be 12 characters long.
AmberNet allows a nickname registration via nickserv. Via a registration, you will be able to 'own' your nickname. Also on your return, it's yours.
On other networks, which have no nickname registration, it's the first come, first serve rule.