If you're not a programmer then, you might want to try an engine first because coding your own, while more fun, will take you a while. At least in C++.
There's a bunch out there but, I would say one of the easiest ones would be Reality Factory. It's free and I think you can do commercial with it if you leave the Genesis Logo splashmark on. But, check it out for yourself, anyhow.
Another route to it would be to mod an existing game as it would teach you a lot about media creation and scripting to tweak things around a bit. In this case, you might want to try Assault Cube. I never did a mod for it but the demo I downloaded was good.