What you will need:
The only thing required is the IWAD for The Ultimate Doom. If you want to play with mods, I recommend Doom 2, or even better, buy them both. The other option is to download them. Google for the links.

b-but Chloe! Isn't that the P word?
Well technically, yes. But nobody who worked on Doom/Doom 2 will get the money from your purchase, it all goes to the big corps. Also, John Carmack said he doesn't mind. However, the choice is up to you. Im just the messenger.

What sourceport should I use?
Its really up to you. If you want to play most modern mods, I recommend using GZDoom. It will support most of the mods you want to play, so it should be fine. If you want to play multiplayer, use Zandronum. Its not as advanced as GZDoom and doesnt support as many mods, but it has decent netcode that works for multiplayer. If you want to play like they did but in '94, then you should use Chocolate Doom. It doesn't run a lot of mods, it aims to recreate the experience of playing the OG as close as possible, going all the way to replicate bugs and glitches.
Where do I get these ports?
zdoom.org
zandronum.com
chocolate-doom.org

How do I play mods?
Theres a couple ways, with the easiest being click 'n dragging the mods you want to play onto the icon of your sourceport of choice. You can also use a mod launcher, however im not entirely sure of what ones work well.

What are some good mods/maps then?
Well this depends on what you want. Google is your best friend here, however some mods / maps I can personally recommend are:

DOOM 4 Vanilla. This mod attempts to recreate the gameplay of DOOM 4 in Doom 2. My personal favourite mod.

Reelism. In this mod, your pitted against countless demons each round, with modifiers applied each time. One round you may be playing vanilla doom, and then the next you will be gunning down nazis using the arsenal of Duke Nukem himself.

HDoom. This mod is far from good, but for those extra weird guys it exists. In this mod, the demons have all been altered to be *cough* seductive *cough*. To those who do want this, you know corn is free right?

What about maps?

Ancient Aliens. This mods hard. Like really hard. But its fun. With neon colours and beautiful architecture, this mod is by far my favourite.

UAC Labs. This map is simple but was neat for its time, and was far from bad. Why is it on here if its not that noteable then? Because it was made the Eric Harris, one of the columbine shooters.

Nuts. By far the most infamous jokewad, this map as you fight against as many demons as the game can physically handle. good luck.

Still not enough for you? Google. There is 20 years of modding out there, all waiting for you to play.

Now, rip and f***ing tear. Until it is done.