You need to identify which IWAD is required to play the mod. The Doom series contains four different IWADs: Ultimate Doom, Doom 2, TNT: Evilution and Plutonia. Most Doom mods are made for Doom 2 but some were made for one of the other games in the series. The other Doom engine games, Heretic, Hexen, Strife and Chex Quest, each have only one relevant IWAD (although registered Strife, along with the map IWAD, also has a voices WAD). You cannot normally run mods with the shareware or demo IWADs.