IWads:
iD_tech_1_doom.zip
https://mega.nz/folder/QXIk1bjB#NHRWGIfjcc77PyGgRlga4Q/folder/dP4GBQhZ

Download Zandronum:
https://zandronum.com/download

Hosting a server:
1. Go to https://allfearthesentinel.net/
2. Register an account.
3. Click Host Server.
4. Enter in the server details (name, gamemode, time limit, maps, flags, mods, etc.)
5. Click Host
6. Announce that your server is up on both /vm/ and /vr/'s Doom thread

DMFLAGS explained:
https://zdoom.org/wiki/Zandronum:_DMFlags

Where to find mods:
https://www.doomworld.com/idgames/
https://www.doomworld.com/forum/4-wads-mods/
https://forum.zdoom.org/viewforum.php?f=43

If you find a mod you want to host a server for, search for it on allfearthesentinel before uploading it there.
https://allfearthesentinel.net/zandronum/wads.php

Make your own maps:
Ultimate Doom Builder
https://forum.zdoom.org/viewtopic.php?t=66745
Eureka DOOM Editor
http://eureka-editor.sourceforge.net/