As the above states, if so where do i start?
Yes you could!
I have personally never played doom, but it is very possible. if your new to unity you should should read some tutorials or create a simpler game before moving on to something bigger. I would try making a 2d sidescroller. If your looking for tutorials, there are plenty out their. Just google it.
If your looking for something more rpg orientated, try burgzergarcade on youtube.
Hope this helped!
you can create most any type of game in any game engine. Start simple even though I have to admit fps games are amongst the easiest to create. Take a look a the unity fps sample project. If you are talking about the original doom, just disable the camera look up/down features.