In options (alt+ o , or second icon from left at the top) there is the top most listing, "Connect" then under it is "Servers"...... in there you can add and delete the servers you want to do either to. mIRC comes with a lot of them in there already so look and see if the one you want is already there..... if not use the "ADD" button.
Description can be anything you want(try and keep it simple to avoid confusion).
IRC Server will be your IRC server name: example irc.network.com .
Ports will be a predetermined port or a port range: example 6999 or 6667 - 6999
Group will be a group name for if say there are 7 servers on the network , add all 7 server names to 1 group.
example network.com and network.net and network.edu could all be listed in NetworkRandom group that you would create. You would simply add each one and where it asks for group just put in NetworkRandom as the group name and they will all be in that group.