How would I go about doing this?

Use the UserInputService.

(I can't edit the page just yet, and they removed the link to the input object, so click here for it. It's the 'touch' or 'input' Instance returned by most of the events.)

--LocalScript
local uis = game:GetService("UserInputService")

uis.InputBegan:connect(function(inst) --inst is the InputObject
    if inst.UserInputType == Enum.UserInputType.Keyboard then --KeyDown
        if inst.KeyCode == Enum.KeyCode.I then
            print("I was pressed")
        elseif inst.KeyCode == Enum.KeyCode.O then
            print("O was pressed")
        elseif inst.KeyCode == Enum.KeyCode.LeftShift then
            print("Left shift was pressed")
        elseif inst.KeyCode == Enum.KeyCode.Two then
            print("2 was pressed")
        end
    end
end)

uis.InputEnded:connect(function(inst)
    if inst.UserInputType == Enum.UserInputType.KeyBoard then
        if inst.KeyCode == Enum.KeyCode.I then
            print("I was released")
        end --I think you get the point.
    end
end)

The InputChanged event can be used to catch mouse movement (though you'll need to capture a Mouse object to get the CFrame it's pointing at) and scrolling, and the myriad of Touch events are all useful for mobile games.