So I am still pretty new to DnD, my first experience was in 2015, and I didn't play again until 2019. I have yet to finish a campaign because of life, COVID, etc. However, I have always had a desire to DM. I love to roleplay and love storytelling. However, I am very anxious and nervous about dm'ing. From my experience some people are looser, some are stricter, and some don't care either way.

I want to build my own towns, world, etc, but is that against the rules? Like, I want the existence of the races, classes, towns, cities, etc from the books (I haven't read any of them) but I want to build my own world around it. Is it frowned upon to do so? Would it be terrible to create NPCs whose race are primarily from other worlds like Eberron etc?

I keep backing down from DM'ing because I am afraid to fuck it all up. Any advice would be awesome.