This command will change your nickname to a new one that you select. This will update all the rooms that you are on. The example below would change your nickname to lucky. Nicknames are limited to 30 characters although some clients may have difficulty understanding more than 9. If so, use /quote nick <nick> or whatever that client requires.
Example:
/nick Lucky
Your nickname is the first thing that other people will notice about you. It is your name and your identity on the chat network. You should try to pick one that you are happy with. You may have to try a few before finding one that someone else hasn't registered yet. You can change your nick to reflect your mood or to show that you are away (e.g. /nick Lucky-away) although you can also use the away command.