You don't need any addition software other than what comes with your OS if your making a script or some other basic addon. You make the scripts and stuff via Notepad/whatever you use for coding. Unless you're modeling something/making music/sounds, you shouldn't need any addition software, but if you are modeling something, blender is pretty much the best free one, and audacity for sounds, which is also free. I'd recommend notepad++ for anything serious with scripting, which again is free.