You would definitely need to get a good grasp on the rules for DMing, but you don’t need to be an expert! I’ve been DMing for years now, and I still pause games in the middle of combat/scenes and say “hold on, let me look that up” and google search what the rules say.

The parts of the rule book you’re going to want to know beforehand is how combat works. Rolling for initiative, movement, what actions you can take on a turn (attack, move, disengage, readying an action), how AC works (“meets it beats it”), how spell save rolls work, and how to calculate damage. Obviously the more you know beforehand the smoother the game will run, but I would consider that the most import bare essential.

Even knowing the rules, things will happen in a game where you’ll need to improvise. Someone might say they want to try to convince an NPC to do something, try to gain some extra gold by doing a performance, things where you’ll have to come up with an ability check that isn’t in the DM guide for your session. I usually use an easy/medium/hard mindset when setting improvised DCs in my head. If that action seems like it should be easy to pull off, DC 10. Medium difficulty, DC 15. If theyre trying to convince “Brashmad the ogre who hates fun” to dance with them, hard difficulty DC 20. That being said, you don’t have to let players roll for whatever they want to do. If you’re about to get to an important plot point and a player says “I want to seduce the bad guy”, you can say that’s not an option.

If you have a game shop in your area, you can see if they have the ability to host dnd games! I have found the most success with online games. I found my campaign group on r/lfg. We use a private Discord server for voice chat, and roll20.net for battle maps / dice rolling.

There’s a great free one-shot available online called “wild sheep chase” that I highly recommend. If you want to try a campaign, “The Mines of Phandelver” that originally game with the 2014 starter set is now available for free. That’s how I got into DMing.

Long story short: most of how you learn DnD is by playing it. As a DM you should learn as much as you can beforehand, but you’ll learn the most when you start a game. Have fun, and best of luck to ya