If you have previously registered your nick with the network's services (such as Nickserv), then you need to identify whenever you connect to that network.
On many networks, you can just use: /identify password
If that doesn't work, you can try: /msg nickserv identify password
If neither of those work, try the network's #help channel. Many #help channels will have information on registering nicks right in the topic, but if not, just ask in the #help channel.