That sounds like a very simple concept, could be funny though, at least for half an hour or so. Just the right thing for a total beginner.
Do you want that do be a first person shooter?
The first thing to do is to plan what weapons should be there, what enemies, how should the levels look like, stuff like that.
You surely don't need more than a few pages to write all details for your project down.
When you have that done, come back and start asking more detailed questions. First, do the workshops from the download page.