Start an idea bag.
I used to collect into a 13-pocket file folder everything I could get my hands on that was interesting or cool about D&D adventures (it was pre-internet when I started this). I kept pockets for different pictures of people (NPCs), places, things (treasure), monsters, and maps. Lots of pictures. I also kept a pocket of NPC names (always useful), city and place names (again, very useful), and a collection of personalities that I could draw on to suit my needs (typically a few bullet points; never too much detail).
Besides pictures and lists, I would start by going through the Monster Manual, and spending an entire day coming up with story ideas, motivations, special places, etc., for each monster--that's one monster a day. Throw away your first 10 ideas, because they are the most obvious ones...stick it out for that odd 11th, 12th, or 19th idea. Those are the ones that will make you seem like a genius to your players. Write those down in a journal, or cards, or whatever works for you, but do the exercise.
Anyway, those ideas, and collecting them into one place, you'll find that your mind has a way of remembering most of what you collect...when all those ideas are crashing into each other inside your head, that's when the real magic happens.
This is also why experienced DMs can make it look so easy. They've already digested a lot of ideas from their previous 10, 20, or 30+ years of gaming as either a player or DM. It is almost effortless for them to pull up past ideas, vs, a newbie who has never invested that kind of thought or even been exposed to the creative process.
It's only because of my idea bag, that I could step up to any gaming table, and whip up a "quick one-shot" in about 5 minutes on-the-spot, even without touching my physical idea-bag. Now, did I really just create an awesome adventure in 5 minutes? Yes, and no. Yes, I did, but there was an investment of prep into my idea bag that made it possible for me to create that adventure in 5 minutes. That's what newbie DMs don't get. It's not just improv. It's improv with a serious investment of time and energy that got poured into it well beforehand.
So, just start by collecting ideas, and start building your idea bag with everything that is within your reach. The only DMs that fail at being creative are the ones that don't invest time and effort into themselves, and the creative process.
PS. Would people be interested in a dedicated Twitch stream on homebrew ideas? Campaigns, scenarios, NPCs, monsters, custom treasure, etc? I keep thinking the D&D streams that interest me most are always the DM creativity and planning ones, and less so the actual play ones, but I get it will be a small audience since it is geared only for DMs. Just curious. My computer is old, so I've not jumped into it, but I'm also on a break from work until April, so this is a great time for me to try it out.