DOOM was released originally on PC for early 90's computers. It has been ported to many
consoles to follow. Since it's original release on PC, especially after the release of it's source
code, many highly modified versions allowing more levels goods and enhancements were made. These were called
source ports. If you are NOT running a MS-DOS or Windows 95 computer (which more than likely you are not
anymore) you will need to pick one of these and stick the DOOM.WAD or DOOM2.WAD file in its folder.
You can also provide TNT.WAD, PLUTONIA.WAD, HERETIC.WAD, HEXEN.WAD, STRIFE1.WAD, and
many other free IWAD files such as CHEX.WAD, the free five-level shooter that came with Chex cereal in the 90's, or FREEDOOM, the free version of Doom using alternate resources enemies and levels to not break copyright law.
The previously listed pay IWAD files for original Doom and clones are available on Steam for just a couple bucks, or you can find "other" ways to get them, if you are a pirate.
Upon doing this, you can play more modern modifications and megawads, including those with high resolution textures and floors, skyboxes (skies that wrap completely around and don't just loop), MD2 Models instead of pixellated enemy sprites, MP3 Music instead of the traditional MIDI, and many other helpful modern enhancements, such as floors above floors and slopes, and unique programming languages to do whatever you want in-game. The two that I use are ZDOOM and GZDOOM