This is pretty easy to do.

Type /msg nickserv register password email (replace password and email with your password and email address).

After that you will be sent an email to the email address you provided. Copy/paste the portion it tells you in the email into mIRC.

Once nick is registered all you need to do every time you come onto the dejatoons' network is type /msg nickserv identify password (of course password is whatever you chose for your password).