https://wiki.freenode.net/view/NickServ

The following commands allow for registration and maintenance of nicknames; to use them, type /msg NickServ command.

REGISTER password email

Registers your nickname in the NickServ database. Once your nick is registered, you can use the SET and ACCESS commands to configure your nick's settings as you like them. Make sure you remember the password you use when registering - you'll need it to make changes to your nick later. (Note that case matters! ANOPE, Anope, and anope are all different passwords!)

This is assuming that whatever IRC network you're using uses NickServ. I'd guess that some don't.