What you see next is the mIRC Add Server dialog box.
You'll need to fill in some information that you got somewhere else:
Description, usually with the group first, then the geographic area like: Undernet: US, TX, Dallas
If you have regular meetings you can personalize like: Prometheus group meeting
The same server could be listed several times for each of your different regular chats.
Address, like dallas.tx.us.undernet.org This one you'll need to know already.
Possibly the port, but most are at the default 6667 This particular server uses three ports: 6667,6668,6669
Group, like: undernet You'll need to know this one.
You won't likely need to use the password.
We'll pretend you typed this all in.
Now click on the Add button to add this server to the list.