How do you make a GUI pop up on someones when they touch a part?

I'm not very good at scripting, and I don't know much about it, and I don't know how to make a GUI. When I did this, I really took things I know and things I thought were going to work and made this:

function onTouch if 
	game.Players onTouch
	function Display game.Workspace.ScreenGui
end

I don't know if this script would display a ScreenGui when I touched the part. (This script is in the part I'm trying to use).

This script uses the Touched event. Then used the GetPlayerFromCharacter method to make sure that it is a player. Then I make sure the player does not have the gui already using FindFirstChild. Then clone it into their PlayerGui using the Clone method.

Just add the path the the gui.

local Gui = Game:GerService("ServerStorage").???

script.Parent.Touched:connect(function(Part)
local Player = Game.Players:GetPlayerFromCharacter(Part.Parent)
	if Player and not Player.PlayerGui:FindFirstChild(Gui.Name) then
		Gui:Clone().Parent = Player.PlayerGui
	end
end)

function onTouch if 
    game.Players onTouch
    function Display game.Workspace.ScreenGui
end

I think you just made this up...

script.Parent.Touched:connect(function(p)
	if p~=nil and game:getService("Players"):getPlayerFromCharacter(p.Parent) then
		local me=game:getService("Players"):getPlayerFromCharacter(p.Parent)
		if me.PlayerGui:findFirstChild("Message")==nil then
			local msg=Instance.new("Message",me.PlayerGui)
			msg.Text="Text here"
			game:getService("Debris"):AddItem(msg,5)
		end
	end
end)

This will give the player a simple message in the center of their screen for 5 seconds.

debounce = false
script.Parent.Touched:connect(function(hit)
local player = game.Players:GetPlayerFromCharacter(hit.Parent)
if player and not debounce then
debounce = true
local gui = Instance.new('ScreenGui')
gui.Parent = player.PlayerGui
local frame = Instance.new('Frame')
frame.Parent = gui
frame.Size = UDim2.new(0, 50, 0, 50)
debounce = false
end
end)

You can edit the GUI properties from there. Use this as a reference for the Frame properties:

http://wiki.roblox.com/index.php/RBX.lua.Frame_(Object)