Use your primary nick
If you have multiple nicks associated with your account, use the primary nick for the account. If you're unsure which of your nicks is the primary nick, type the following command into IRC to show the account name:
/msg NickServ info <yournick>
Register on IRC
You can register an account on IRC itself, by using the following command and following the instructions.
/msg NickServ register <your-chosen-password> <your-email-address>
Note: Passwords cannot contain spaces, and you'll need to use a valid email address and verify it to complete your registration.