How can I make the User UserInputService grab the "e" button? Any tutorials?

This would actually fall into using the Mouse and PlayerMouse properties instead of UserInputService.

Here is some sample code that prints the key pressed to the output: (note: it must be placed inside a LocalScript, which in turn is placed inside of the StarterGui)

local plr = game.Players.LocalPlayer
local mouse = plr:GetMouse()

mouse.KeyDown:connect(function(key)
    print(key)
end)

--Written off the top of my head, may or may not work.