How can I make a Gui show when I press a certain key?

I personally think reading the wiki is the best way to learn more about something as many talented people give a full explanation of each script.

Right here is the wiki link to taking keyboard input http://wiki.roblox.com/index.php/Taking_keyboard_input

Just use what you learn there to make a function that turns Your gui visible to true.

there is honestly a bunch of options on how to do it so you pick what best fits you!

Also if you're new to roblox lua you should probably watch some tutorials as they are very useful!

This might work,

game.Players.PlayerAdded:connect(function(mouse)
mouse.KeyDown:connect(function(key)
if key == "the key you want the player to press"  then
game.Players.LocalPlayer.PlayerGui.GuiName.FrameOrWhateverName.Visible = true
end
end)
end)

EDIT: Put the script in workspace