When you open mirc there, a dialog box opens up titled "mIRC Options". Close that box by pressing the "Cancel" button.
Now there is one window open, labelled "Status". Click on the area where you write the commands and write:
When it is connected to the server, write:
to join the chatroom.
To change your nick, use:
and if it is registered don't forget to sign in with NickServ using
/msg nickserv identify YourPasswordHere
Hope this helped a bit