Registering your nick on a network varies from network to network. If they have nickserv, you can use "/nickserv help" to get a list of commands (such as REGISTER and IDENTIFY). For additional help on whatever network you are on, join the #help channel and ask them how to register your nick on that network.
And, after you register, you will need to activate the account (check your e-mail for instructions) and then you have to log in (IDENTIFY) to it whenever you connect.
