Wait, back up a step, have you heard about the DM's Guild? It's a website that allows ANYONE to publish adventures.
You can find a variety of adventures from fun homebrew to gorgeous professionally produced epics - and generally for very low prices (most of it is free to $5). You do have to create an account to buy or publish adventures.
Now For Those Pesky Details
The DM's guild allows you to sell PDFs of D&D adventures (as well as classes, magic items, and so on) that you write. Wizards of the Coast (WotC, the company that makes D&D) and One Book Shelf (OBS, the company that runs the DM's Guild website, as well as Drive Through RPG and other game PDF vending websites) split half of the proceeds and you get 50% of the proceeds.
You can publish material that has no setting or uses the Forgotten Realms, Ravenloft, Eberron or Ravnica settings. You can also use Wizards of the Coast Intellectual Property (IP). That means that Elminster, mind flayers, owlbears, and everything in FR, Ravenloft, Eberron, and Ravnica are fair game for you. These things are all WotC IP and they don't let other people use them without a special agreement - and one of those special agreements is with people who publish on the DM's Guild.
You retain ownership of your work, BUT anyone else who publishes on the DM's guild can use your work if they attribute it to you. If your adventure has a cool town in it, they can write an adventure in that same town. Or use one of your NPCs.
Once you publish it on the DM's Guild you can't publish it anywhere else. They don't want to compete with other websites.
Where Do I Start?
The first thing, the VERY FIRST thing which you MUST do is read the DM's Guild page about how to get started.
https://support.dmsguild.com/hc/en-us/articles/360024677993-Getting-Started-on-Dungeon-Masters-Guild
What Next?
Read the DMs Guild Rules for content creators. Yes, it's a lot of reading but you need to know the rules before you get too involved here.
Buy and read three top-selling products similar to what you want to create. What template did they use? How intensive is their art design? What sort of things did they include in their product and did you think about them for yours?
Layout and look are important. We can all agree that while you probably shouldn't judge a book by its cover, it is nonetheless something that people do. Pretty products sell better.
Find legal art. The DMs guild sells art packs that are designed for people to use for DMs guild products. You can also investigate free public domain art. (Finding art on the internet DOES NOT mean it's public domain.
Here is the DM's Guild Guidance: https://support.dmsguild.com/hc/en-us/articles/217029188-Logo-and-Artwork-Questions
And here's a blog post with some more excellent options:
Format Is A Thing. The DM's Guild provides a format for you to start with. A lot of products use it and a lot of products don't. Me personally, I hate it but your mileage may vary. I've published plenty of stuff in this approximate format and they've sold well.
Cartography Is A Thing. Pretty maps sell. Fortunately, Dyson Logos allows anyone to use a specific selection of his maps, for free, in print products with proper attribution. Check out his generosity here:
Start Small
While it is SO TEMPTING to go for broke and pen a 130-page epic adventure there are two problems with that.
1) That's not what sells best on the DM's Guild.
2) Writing a 130-page epic adventure is REALLY hard.
Start small. Write a single encounter and post that. Write a one-shot adventure that will take a couple of hours to play. IDEALLY, you've already bought some products from the DM's Guild like this so you have some basis for what these things look like.
I've written several 5,000-20,000-word adventures that run up to maybe 20 pages. Let me assure you that's hard as hell. Practice a few of those before trying to write your opus.
Keep Your Ears Open For Opportunities
There have been some FANTASTIC products written lately that have solicited writers on social media (primarily Twitter) like The Lonely Scroll, Uncaged, and most recently Eat the Rich. Keep checking out social media for writing contests and anthologies that are looking for authors. This is a fine way to write a small adventure and have someone else deal with the realities of the DM's Guild.
Writing Advice
There are a whole lot of people who have a whole lot of writing advice. Here are some that I strongly recommend.
How to Write Adventure Modules That Don't Suck by Goodman Games
See P. XX: 5 Tips to Sharpen Your RPG Prose by Robin D. Laws
Read EVERYTHING Shawn Merwin has written about writing adventures. Then do what he says. That's easier said than done but do it.
Writing in the Forgotten Realms is special. There's a whole set of expectations that people have that are built on many years of playing the adventures and video games and reading the books. Here is the DM's Guild guide for writing in the Forgotten Realms. Definitely read it even if you're not writing in the FR.
Sly Flourish has some great material as well:
Price
I have two pieces of pricing advice.
1) Do not give your stuff away for free*. Always put a fair price on it.
2) Check to see what everyone else is charging and set your price accordingly.
* While you should not give your stuff away entirely for free, you should give SOME INDIVIDUALS free copies for advertising.
Some thoughts on DMs guild pricing
Advertise, Advertise, Advertise If you write the best adventure in the world, and no one knows about it, then that adventure effectively doesn't exist. You will need to actively market it. This is part of the publishing process and one that FAR TOO MANY authors hate and just don't do. Which is fine. You don't have to do it. But if you're doing this to actually make money, then you need to go all-in with your advertising.
Talk about your adventure on social media. Get your friends to do likewise. Post something about it every week for the first couple of months, and twice a month after that.
Send free copies of your adventure to local conventions so they can run it.
Send free copies of your adventure to online reviewers.
Run contests to give away copies of the adventure on Twitter.
Find podcasts and let them know you'd like to be interviewed about being a new author.
Find a stream that needs content and run your adventure for that stream.
Run your adventure in the local game store or convention.
Give away copies of the adventure on social media BUT ask the recipients to leave a review on the DM's Guild. People are leery of buying an adventure with few or no reviews.
Do Your Homework
There are a lot of people with a lot of opinions about publishing your first adventure. Go read up on what other people are suggesting as well, and then do what's best for you!