Code:
on *:text:!addcom *:#: {
  if ($nick isop #) {
    var %r = $read(# $+ commands.txt,ns,$2)
    if (%r) { .msg $chan [ $+ $nick $+ ]: Error, This command $qt($2) is already exist into the database! | return }
    write # $+ commands.txt $2-
    msg $chan /me + Command $2 has been added to the database!
  }
}
on *:text:!delcom *:#: {
  if ($nick isop #) {
    var %r = $read(# $+ commands.txt,ns,$2)
    if (!%r) { .msg $chan [ $+ $nick $+ ]: Error, This command $qt($2) does NOT exist into the database! | return }
    write -dl $+ $readn # $+ commands.txt
    msg $chan /me - Command $2- has been deleted from the database!
  }
}
on *:text:!editcom & *:#: {
  if ($nick isop #) {
    var %r = $read(# $+ commands.txt,ns,$2)
    if (!%r) { .msg $chan [ $+ $nick $+ ]: Error, This command $qt($2) does NOT exist into the database! | return }
    write -l $+ $readn # $+ commands.txt $2-
    msg $chan /me -> Command $2 has been updated!
  }
}
ON *:TEXT:*:#: {
  tokenize 32 $strip($1-,burci)
  if ($read(# $+ commands.txt, nts, $1)) {
    var %com = $v1
    var %com = $replace(%com,@user@,$iif($2,$2,?),@nick@,$nick,@target@,$target)
    if (-ul=mod == $gettok(%com,1,32)) && ($nick !isop $chan) { msg $chan  | return }
    msg $chan $iif(-ul=mod == $gettok(%com,1,32),$gettok(%com,2-,32),$gettok(%com,1-,32))
  }
}


This is very basic, all I did was rename the file from commands.txt to # $+ commands.txt which will create a new .txt file for each channel. (Which is exactly what Belhifet suggested)

To be able to make a script you need 2 things:
Knowledge in the language
An idea

You can get the first one from reading the help files: type "/help" in mIRC as well as on this website.
The idea part, well, that's entirely up to you. Can't really help you there.