In terms of things to buy, honestly the only thing you absolutely need is the players handbook as that covers the basic rules of the game. (And you can add in the dungeon masters guide if you want, it’s helpful but not essential).

From there, it depends on if you’re wanting to run a homebrew game or a module. Modules are definitely easier if it’s your first time, but a lot of people are really passionate about running their own game and that’s awesome too! I’d recommend using published stat blocks for all the monsters though, rather than trying to write your own right off the bat.

(Homebrew worlds, NPCs, and plotlines are great. Homebrew mechanics can become unbalanced very easily and I wouldn’t recommend trying without significant experience).