In my experience, the best way to prepare for running any RPG is to first have someone run the game for you. That can be hard, but the good news is, D&D 5E is the most widespread TTRPG in history, so finding a game shouldn’t be too hard, especially since you’re already talking online games. (Some areas are TTRPG deserts, but finding a one-shot 5E game online is as easy as it gets.)

If that doesn’t work for you for some reason, I would suggest the following:

• Run a prewritten adventure, but don’t worry about sticking too close to the book. You’re already doing this, so good thinking! A published scenario (by that I mean, a short adventure, not a full campaign) does a ton of work for you. The one thing I would caution is trying to stick too close to the way the adventure unfolds. If things veer off in an unexpected direction, just go with it.
• Find a D&D 5E cheat sheet for you and your players. It’s easy to forget a rule in the heat of the moment. Don’t worry about getting “everything right” and stopping to look up a rule. If something isn’t on the cheat sheet, just tell players you’re going to make a ruling in the moment, move on, and look it up online later. Unless a character is literally on the verge of death, it probably doesn’t matter anyway.
• Make a one page of prep notes for the adventure. Distill all the major NPCs, locations, treasures (if applicable) on a single sheet of paper. That way if someone asks, “Wait, who’s the mayor again?” You won’t have to dig through the adventure, you can glance down and say, “You mean Fuzwibble McShiddlebins, the Tabaxi male Cleric?” and watch their jaws drop and have everyone pronounce you Best DM Ever (this won’t really happen, but it sure beats trying to find NPC info in the heat of the moment.)

Finally, I’d like to share a few thoughts about what it means to “DM effectively”. Running a game is an art, not a science. Whole books have been written on the subject, and no two agree on most things.

The best thing to keep in mind is: everyone is there to have a good time. Yourself included! Players want to do cool things. The DM wants to thrill and excite and entertain their players. Mistakes are going to happen, but don’t let them live rent-free in your head. It doesn’t matter if anyone gets rules wrong, especially when you’re all still learning. It’s been years and I still can’t remember certain 5E rules (passive perception and darkvision, yikes). My group still has a fun time playing, because the game runs smoothly enough. Things like safety tools, cheat sheets, one page prep notes, all this stuff is designed so you run into as few roadblocks to fun as possible at the (virtual) table.

Good luck, and feel free to ask any followup questions if need be. :)

P.S. I think this might have been posted in the wrong place? It probably belongs in c/dndnext, c/rpg or c/askgamemasters.