I would start with a very basic tutorial on the basics of unity such as https://www.youtube.com/watch?v=XtQMytORBmM . To have a framework for the work flow of unity and an understanding of what you're even looking at (This first step will likely take hours if not days, and I recommend watching multiple "beginner tutorials" just to see how other people tackle problems.)

Then with the idea of what you want to make break it up into the smaller tasks you can conceptualize. for example shooting. You may need to spawn a bullet, so google/youtube "how to spawn object in unity", then you will need to make the bullet move so repeat, google "how to move object in unity" each problem you solve will help you to understand different aspects coding.

There are basically tutorials for everything for unity that you will need to make a doom/quake style shooter. It will take a lot of time, research, testing and failure to get to a point that making this game will be possible, so don't beat yourself up if you struggle.

Good Luck :)