Well first you need to get some doom wad files, these are not included in retropie.
They need to be copied to RetroPie/roms/doom. then when you run emulationstation you can select the doom wad you want to run and it should work.(don’t select the prdoom thats already there, it doesn’t do anything).
The easiest way to get the roms copied onto the pi is to insert a blank formatted usb memory stick into the pi usb with emulationstation running. It will create a rom folder on the usb stick, with sub directories for all emulators.
Remove the stick and plug it into a pc, where you can copy the wads ( or any other game roms ) to the appropriate folder. plug the usb back into the pi and it will automatically copy the roms to the correct folders for you.