How to ignore all JOINS, PARTS from all but a few channels

auto connect multi-server: Menu "Tools" > "Script Editor" > Tab "Remote" add somthing like this:

on *:START:{
  server irc.network1.com
  server -m irc.network2.org
  server -m irc.network3.net
}

change your nick by network: Menu "Tools" > "Options..." > Category "Connect\Options" > Button "Perform..." > Check "Enable perform on connect" > Choose (or Add) your Network > Add the following to the text box:

/nick yournick

join a channel (still in the same box):

/join #yourchannel

and as stated by Idigas, you could add any mirc commands there

(There's an alternative to auto join a channel: Join your channel > Menu "(Tools >) Favorites" > "Add to Favorites..." > Check "Join on connect" > OK)

update: you can also do everything, adapting the auto-connect script: server [-m] irc.network.org -i yournick alternativenick -jn #yourchannel1,#yourchannel2

and to connect to any server of a network group, simply replace the server by the group name: e.g. replace "irc.freenode.net" by "freenode"

("/help /server" to get all the /server switch)

Set your nick password as the server password. Freenode's IRCd accepts the server password and will attempt to identify with it.