Moderator and Admin commands?

Hi. If you want to run specific code whenever someone says something, you should look into implementing the Player.Chatted event.

game.Players.PlayerAdded:connect(function(player)
    player.Chatted:connect(function(msg)
        -- do stuff with msg and player
    end)
end)

You can then read the message and Player instance for each message. You might only want to allow certain players to run admin commands. For that you might want to do something like this:

local admins = {
    "frederikam",
    "kannsin34",
    "Player1",
}

local isAuthenticated = function(player)
    for i,v in ipairs(admins) do
        --Read the list for our admins
        --Compare our player to the admins
        if player.Name == v then--The player is an admin
            return true
        end
    end
    return false--Only run if the player's name did not match
end

Edit: I also recommend looking into http://wiki.roblox.com/index.php?title=Using_the_Chatted_event

Adding on to the other guy's answer, this could double as a admin list or a ban script.

list = {["Person"] = true, ["LordDragonZord"] = true, ["Haker"] = false} --false is ban, true is admin, nil is neither

function admin(n)
    return list[n]
end

game:GetService("Players").PlayerAdded:connect(function(plyr)
    if admin(plyr.Name) then
        plyr.Chatted:connect(function(msg)
            if msg == "" then --If they say...
                --Code
            end
        end)
    elseif not admin(plyr.Name) then
        plyr:Kick("You've been banned") --Edit the message as you please.
    end
end)

Hope it helps!