"If you want to become the DM, first you must watch online video tutorials"
-Somebody Smart I Guess
Being the Dungeon master is probably the most rewarding and stressful job even invented. The DM, as I have previously mentioned in "The Basics". It's the DM's job to make the world, play as all of the supporting characters, the monsters, and make a story. "Great", you think, "why don't I just write a book?" Well, my innocent new DM, a book can't subvert your expectations like real players can. A book can't decide to become mercenary for the villian for money, or ignore your plot points that you spent hours making in favor of a random goblin named Gwizzlet that you just made up on the spot. If that sounds fun to you, I highly recomend watching a lot of video tutorials on Yotube to learn the basics (see below).If you stil want to be a DM, you will need the following.
As all of these things can be found in the "Lost Mines of Phandelver" 5e starter kit, so it's reccomended. However, notice none of those things require that you spend money. You can try DND with some friends without spending a cent, and that's cool.
Below is the first video of Matthew Colville's DND series "Running the Game." It's commonly reccomended for new DM's. The first five episodes are a pretty good summarry of how it works to DM, and it's worth a Youtube search.
Also, if you don't know what Critical Role is, you can check it out here. (External link warning)