From LegendWiki

Changing Your Username

If you need to change your username for any reason, type /name NewName, replacing "NewName" with the username you want to use.

Registering Your Username With NickServ

Registering your username on IRC ensures that no one else can log in under your name. To register the username you are currently using, type /msg nickserv register password email, replacing "password" and "email" with any password you choose to use and a valid email address. Within a few minutes, you should get an email containing a passcode. To complete your registration, type /msg nickserv confirm [passcode]. Your username is now registered to you, and can only be used when you know the chosen password.

Setting Up AdiIRC to Auto-Join and Identify