Some networks have Services such as 'NickServ' which allow you to have ownership of a nickname. In order to register you need to use certain commands with the network's services, the most common format is /nickserv register password email@address but this by no means applies to every network.
Services have nothing to do with mIRC, so for further help with network services you need to contact the network regulars/helpers who are often in a channel like #help, or check the network's website.
Regards,
