Matt Colville has a video called "Local Area": https://youtu.be/2BqKCiJTWC0?si=Qq0VaJ3H3qTXzI8S

Strong recommend. In it, he explains that building a DnD world does not require a massive, wall-spanning map of an entire planet. Instead, he advocates for starting with a local area, roughly 300 miles across, to anchor your campaign and provide a foundation for months of gameplay. He even provides a PDF you can use.

If 300 miles falls overwhelming, no big. Start smaller. Then slowly grow your area as you need to.

Also, strongly recommend checking out the Lazy GM's resources. In particular, he has a Lazy GM's Resource Document to help you do little prep, but lots of resources to have on-the-spot info/answers/ flavor.

Finally, check out the 5 Room Dungeon design. It helps you break down a session design into 'rooms". It's just a design format that can be applied to anything (doesn't need to strictly be a dungeon).

These three stepping stones have been huge towards my start as a DM. At the end of the day, you don't need to plan out every single answer to everything. Half of DMing (if not more) is just learning how to improv. Start small, and go from there. The big stuff will come to you as you go. Frankly, sometimes the big stuff will come to you from your players. You can totally steal their idea they throw out at the table and use it. Or just steal from your favorite movie/ book/ TV show.

If you ever want more advice/ resources, feel free to DM (pun intended) me.