Basic Outline of an autojoin script for mIRC

there are two methods you can use
all parts marked with <> are values you need to change
1) Using perform in the options

Options -> Connect -> Perform
Then add the following lines

/nickserv identify <password>
/join <channel1> <channel1pass>
...
/join <channelX> <channelXpass>

2) Using remotes (alt+r in mIRC will bring up the remotes section)

on *:connect: {
nickserv identify <password>
join <channe1l> <channel1pass>
...
join <channelX> <channelXpass>
}

Note leave out the <> and the pass's are optional