If you mean the Undernet 'X' service, then read the channel registration Acceptable Use Policy
(AUP). Then login at http://cservice.undernet.org/live
and click on 'Register a channel'. If you get the message that the server is too busy then try later.
If it's another network, ask in their #Help channel or check out their website. If you don't know the website address, search Google
By the way, we mainly help with mIRC the IRC client here. There are many different networks and some of them have 'ChanServ', 'Q', 'L' or no service at all for channels, they do not all have 'X'. Therefore, it's best to ask in the network's help channel or check their website for future network-specific information.