1) Regarding using mIRC to download:
Read this sticky2) Regarding registering a nick. On most networks, type /msg nickserv register <password> <e-mail address>
2a) Replace <password> with a password of your choice. On some networks this is case sensitive.
2b) Replace <e-mail address> with a valid e-mail address that you have access to.
2bi) The e-mail address may be used by the network to issue a letter with a confirmation code.
3) None of this has anything to do with paying for mIRC.