How do you make a GUI so I can tell players things when they click it?

Make a GuiButton and put a local script in it like this.

script.Parent.MouseButton1Click:connect(function() --access the button, then the clicked event, then connect it with an anonymous function.
    --do stuff here whenever the button is clicked
end)

Studying events in the wiki would help you figure other things like this out. That's how one becomes a great scripter.

Well I gave you the firing event. Just put in your code.

--Local Script
plr = game.Players.LocalPlayer
sp = script.Parent
sp.MouseButton1Down:connect(function()
    -- Your code here
end)

Where do I find GuiButton?

function onClicked()
    script.Parent.Text = "Put your text here"
end

script.Parent.MouseButton1Click:connect(onClicked)

Insert that into a TextButton in a ScreenGui.