how do i make something happen when i hit a certain key?

So, you're asking for a keydown event and all you need to do is this.

--In a local script

local Player = game.Players.LocalPlayer --Get the player
local Mouse = Player:GetMouse() --Get the Player's mouse

Mouse.KeyDown:connect(function(key)
    key = key:lower()
        if key == "" then -- in strings put the key for example q
            --What you want the player to do 
        end
end)

This wiki post might be a good help for you: http://wiki.roblox.com/index.php?title=Keyboard_input

And it clearly states that the mentioned KeyDown way of getting Keyboard Input is deprecated and should not be used.

This works in my GUI put into StarterGUI, so hopefully you can find use for this:

local player = game.Players.LocalPlayer
function clicked(Key)
    if Key == "z" and player.Name == "USERNAME" then
        game.Players.LocalPlayer.PlayerGui.GUINAME.FrameName.Visible = true
    end
end
game.Players.LocalPlayer:GetMouse().keyDown:connect(clicked)