Usage: IDENTIFY [<nick>] <password>

To gain full access to a registered nick, use the identify command. After identifying to a nick, the settings of a registered nick can be changed. You do not have to be using the nick to identify for it.

/msg nickserv identify yourpass
/msg nickserv identify yournick yourpass

In general, it is best if you always identify for your nick when you use it. However, if your access list is set to match you, you will not need to. But many rooms will have ident set on to require that you identify before getting status in those rooms. It can often be useful to identify for a nick you intend to use before you switch nicks. Services will remember you as identified for multiple nicks, even ones you have identified for but not used.