usually, it's /nickserv register (password) (email). Sometimes they don't ask for e-mails, but if you entered it anyways, it's all good.
It keeps from having someone else using your nickname. Once it's registered, you'll have to type /nickserv identify (password) everytime you log in.
Or you can be lazy like most people here, myself included, and go into your options: Connect>Options>Perform...>type /nickserv identify (password) in the text block, click ok, and leave your options. This makes mIRC perform that step automatically everytime you connect to a server.
