A copy of Doom2.wad and Ultimate Doom Builder is all you need for now, start there, experiment, make some maps, suck at it till you get good.
After you got mapping sorted out, get SLADE editor, check the zdoom wiki (it'll be your best friend throughout your modding journey) and start writing your own custom entities using original ones as a template, the more you learn to wrangle the code, the more you'll know how to make completely custom stuff as you go on.
I also suggest you to join this discord group called "The Entryway" created by James Paddock of Adventures Of Square fame, there's a lot of sections where you can share your work, ask for people to playtest, ask for help in case you're stuck on something. Here's the link: https://discord.gg/Ascc59b
James also has made a tutorial that teaches you how to make a simple doom map: