If you mean that other people in your house use your running mIRC: hold down ctrl while pressing the minimize button, then you can type in a password to prevent others from doing anything to your mIRC. Also check Tools->Options ->Lock (the last category in the tree view) there you can set mIRC to ask for a password when you startup, so your family cannot open another client to act as you.
If you mean some kind of nickname registration for the network you are on: this is not a part of mIRC, but of the irc server. A number of irc networks offer such services, but since they differ from network to network, it's really best if you check out their web site or ask in their help channel. Usually it's nickserv, but X or Q or something else is also possible.