You aren't retarted or a noob, we all have to start somewhere.
From your drawing, your game looks very simple to do. It could probably be done in a week or even a weekend.
Random spawning is a topic much discussed, as there are various ways to do it. Shooting and moving is also very straight forward.
Your concept is quite general and plain. Now I'm not having a go at you, but try add something interesting, like kill streaks or a reward system or if you are really daring, multiplayer.
The only thing that might take a while is art and the upgrade system, but even that should take about half an hour to an hour, maybe less.
I'm on a mobile device so I can't really post instructions on how to do everything, but maybe later I'll make a tutorial specifically for you.
Have a nice day/night :D