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.

Welcome to the hobby!!

I have a couple of articles that may help you out:

https://slyflourish.com/getting_started_with_dnd.html

https://slyflourish.com/top_advice.html

Enjoy!

To start with you'll want either the basic rules or Player's Handbook. The basic rules are free on DnD Beyond (that's the easiest place to find them, but you can also just Google it). Personally, I say that if you're doing 5e, you should have the PHB or some way to access the player options in there

You'll want to go through the character creation section, skill checks, combat and spellcasting. That should be enough to start. Next, I highly recommend making a character. By making a character you'll learn well enough to teach your players.

And finally, nothing has been more useful to me than Matt Colville's running the game playlist. You only need the first few videos to get started, the rest is optional advice and things he likes to do

Happy you are dipping your toes in. First rule - have fun.

There will be spoilers below. When I have to prepare actual scenarios I tend to start with the end, what the goal is. From there I look at how the adventurers can get there. For this adventure it is the recipe, so locate that and its immediate obstacles.

Then I start from the beginning and read through the adventure to get a feel for its flavour. For example where does it land on the goofy <-> serious spectrum. Make notes of these flavours and try to reinforce them during play, by that I mean to bring them up and/or exemplify them. Such as the grumpy tree outside.

Now time for actual "room" (location) prep. For you the important parts are in the paragraphs under the boxed text. There you find monsters, suggested skill checks etc. But I don't look at those for now, rather I figure out the goal of each location. What do the adventurers get in this location that brings them forward? Location 1 for example. There the adventurers can learn there are goblins inside that doesn't use the front door. And that old Grammy used to talk to the dryads (where are the dryads??? In the orchard) who may know more. Also figure out how they are supposed to learn this (talking to the tree, how is it revealed the tree can talk?). And finally to what other locations does this location lead to, and how.

By doing all that I end up with the adventure reformatted in a useable format for me. The information I need ready to be used.

As for foundry I bet you can find videos floating around. While looking at them slap up a game and roll up a character. Test out the sheet so you know how it functions and can assist the rest of your group.

DnD specific stuff. Make sure your players knows their characters, any casters how their spells work etc. And when in doubt, as in the characters try something risky where failure brings the adventure forward, ask for a roll. Just pick a skill that makers sense vs a DC of 12-15 (adventure seems to be in that range).

I have probably missed things or explained things badly here and there sp just ask any followup questions you have.