that is a really good question.. 🤔
first of all I would advise everyone to not start out with your own engine outright
especially if it's your first game
and try to find a fps creator or roblox or something premade easy to use engine first
no matter what you choose though your biggest hurdle will be to find a decent scene or level editor
and get good or find someone who is good at making maps 🤔
in a way fps games are easy to make - all you need is a camera, some scene and to be able fire bullets at stuff
the tricky part starts when you want something more complicated than that 🤔