Normally this sort of thing requires you to know a programming language, or at least have an understanding of programming logic. I do have a few recommendations however that may help. If you have absolutely no understanding of programming logic I suggest you try playing LightBot, its a great little game that introduces players to the concepts of programming. From there you can move on to Kodu which provides a number of easily assembled logic blocks you can use to make a shooter style game.
If your interested in further learning after that there are a number of other resources available though opinions and tastes vary on which tools people recommend, such as Scratch, Game Maker, and Python, among others. If you have any further questions, feel free to ask.