Unless you are in a channel, or a pm (private message) with someone else, then anything you type in must be prefixed with the / character, so that mIRC knows that you are sending a command.
Without knowing the network, I would suggest you try
If nickserv is on the network, then you will get a response with information regarding registering your nick.
Finally, the best place to get specific help for the network that you're on, is the network's help channel, normally #help
You should be able to join that channel by typing
