um, ok.. well, firstly lets explain, there are several registrations. (see below of definitions)
1. The first is the registration of mIRC, the actual program used to connect to a network. The program MUST be registered as it is [ishareware[/i] at a cost of $20, and can be done so, via the 'How to Register' link on the left of this page.
2. The next registration takes place on SOME networks, but not all, only the ones that support it, the registration enables you to register a nickname so that you are the sole owner of it and no-one can use it without your permission.
3. The next registration is a channel registration which allows you to register a channel so that you are the owner and have control over it.
4. The final registration is registering on the networks website, if they have one and is often not the case.
--------------------------------------
Definitions
network - A network is a group of servers linked together of which users connect to enabling them to chat.
server - A server is what you are actually connecting to when you run mIRC.
channel - A channel is basically a chatting room where people go to collectively talk about a specific topic.
--------------------------------------
So back to the point at hand, you will need to register a NICKNAME, before joining the channel you as seeking to, try typing, /nickserv help register - as that is one of the most common formats and you should get help, if not try joining a help channel on that network and ask how to register your nickname.
Hope this helps.
Eamonn.