Great question!

Yes, I’ve DMed D&D before, although I’ve spent much more of my tabletop RPG career as a player character than as a DM. So the advice I’m about to give is the same advice I’m looking for as I prepare for the next campaign I’m running, and the advice I wished I’d had when I DMed my first game having not done any of this and struggled like hell as a result:

• Look for Resources Online: unlike when I started playing D&D in the early 90s, there are a lot of really good free resources available on the web to guide novice players. A quick google search for “DM advice” pulls up resources from Wizards of the Coast themselves, gaming news sites like Kotaku or the Escapist, blogs and podcasts, forum threads, youtube videos, etc.
• Learn To Manage Information: one of the hardest things a DM has to do is manage information, especially in combat. Every DM struggles with keeping track of initiative order (just saw a cool thing the other day where someone used clothespins labelled with the PC’s names and “Monster #X” or “NPC #Y” etc. arranged in order and then rotated 90 degrees to indicate who’s turn it is), so find a system that works for you. Ditto with monster and NPC vital statistics like HP and AC, Saving Throws and Attack modifiers, spells, etc. 
• Learn to Manage Your Players: another difficult thing is how to manage your players, finding that middle path between railroading and total chaos, moderating personality conflicts, making sure that people aren’t talking over each other or invalidating one another’s actions, making sure everyone’s engaged and happy. I strongly recommend bringing character creation together with campaign creation, so that players feel involved in the story and their characters mesh well with the setting, and so that you can figure out ways to incorporate your player character’s backstories and character drives with the campaign. This will save a lot of headaches later.
• Learn to Tell a Story.You can do everything I’ve discussed above and not make it further than being a merely workmanlike DM if you don’t spent an equal amount of time thinking about how to tell stories. Tabletop RPGs are an exercise in collaborative storytelling, where the DM has the lion’s share of the work bringing the world to live by describing the environment, voicing all of the non-player characters, and arbitering the results of the players’ actions. The key thing is that the DM’s job isn’t to “win” the game by defeating the players, but rather to get the players to feel emotionally engaged in what’s going on in the game, and everything else is secondary to that. Presenting a challenge is only important enough in so far as it makes the players feel a rush of accomplishment when they win. Likewise, the effort that you put into plotting intricate mysteries or showy setpieces is only important to the extent that it makes the players feel emotionally engaged in the world you’ve built. Hence why I suggested above incorporating your player characters’ backstories and character drives into your campaign – treat them as levers for emotional engagement that you can flip to make a story beat “land.”  

But the number one piece of advice I can give:

RUN A PUBLISHED ADVENTURE AND/OR MODULE FIRST. There are almost 50 years worth of published adventures out there, and a lot of the classic ones have been adapted for whatever edition you’re using. These published adventures are the perfect set of training wheels and/or safety nets for a first-time DM, because someone’s already done the heavy lifting of writing out the descriptions of all the characters and environments, of working out all the combat encounters and traps and puzzles and putting all the of the stats in front of you in one place. 

Running one of these adventures means that you can gain the experience you need without having to sweat the details as much, allowing you to focus your limited time/attention on the harder aspects of managing information flow, player group psychology, and storytelling. Once you’ve got that under your belt, you can start improvising on top of the foundations of the written materials, and applying the lessons learned to your own ideas.

Remember, even Griffin McElroy started out The Adventure Zone with a straightforward run of “Lost Mines of Phandelver,” the adventure that comes with the Starter Set for D&D 5th edition….