It would be nice if you actually started looking stuff up, so far, on this page, I see four posts by you, all asking how to do something with your bot. Why don't you try using /help a bit? It's a good place to start to learn mIRC scripting. We certainly don't want to write every script for you.
With that said, you would want to use:
alias randomnick {
var %chan = $1
var %rand = $rand(1,$nick(%chan,0))
var %nick = $nick(%chan,%rand)
return %nick
}
You can use: $randomnick(#channel) to return a random nick from the specified channel. I used a lot of "vars" so you could see how these identifiers work.
If you don't understand part of an identifier, use /help to read more about it.