It sounds like you are connecting to the server successfully, however it is reporting that the nickname you have chosen is in use, and it is then disconnecting you because you are not providing a new nickname. That said, we would need to see the exact message you are seeing in the status window in mIRC to be sure.
The IRC FAQ
answers this (and many other frequently asked questions) about IRC:
34. Someone is using my nickname, can anyone do anything about it?
Some networks support registered nicknames. This means that they allow you to set a password on a nickname so that you own it. Only the person who knows that password will be able to use it. Some networks may not support this, which means that if someone takes your nickname while you are not on IRC, you can message them to ask for them to give it back, but you cannot demand it, nor will IRC operators /kill for nickname ownership normally.
This means that if the nickname you are trying to use is in use by someone else, you will need to use a different nickname. You can change your nickname in the mIRC Options/Connect dialog.
Each IRC network has its own method for registering nicknames or channels, although some do not have this ability. You would need to read the MOTD (Message of the Day, that appears when you first connect to an IRC server) to see if the IRC network supports registration of nicknames or channels. Instructions are usually provided if they do support it.
If you need to find out how to register a nickname or are having trouble registering a nickname on a specific IRC network, the easiest way to get help is to join a help channel, such as #mIRC or #irchelp, on that IRC network and to ask there.