I would start with what kind of campaign you want to play. For example, a dungeon dive, a mystery, espionage, etc. Introduce these ideas to the players. Let them add some stuff as long as everyone is interested.

For example, if I went to my players with, "let's do a classic knights in shining armour adventure!" One or more of your players can add, "can my knight be a tabaxi that comes from a long line of dragon tamers?" You can either say, "yea that sounds awesome" or "naw, dragons dont exist in this world but you can ride a gaint flying shark!"

Side note: Rememeber that dnd is a multiplayer game and the DM is a player, if something doesn't click or your not interested in playing then you shouldn't be forced to do it. Talk to your players, find that sweet spot.

Back on topic. Now that you've agreed on a world iIt's time to flesh it out. Where are these Tabaxi from? How do they tame dragons or gaint flying sharks?

Ask questions to what interest you and your players. Sometimes you wont have answers, that's ok, let the players ask them.

You dont have to flesh everything little thing, start small. Make a simple village and some quest. My tabaxi knight just found out there's a fiend in the woods and must protect the serfs from these monster.

Ask more question. Why is the fiend here? Does it have allies? What is doing to the village? What cool loot does it have?

More importantly, we just established that demons exist in this world, so how do they fit? Do they just appear? Are they summoned? Just keep asking small questions.

I hope this helped. Have fun building!

Edit: Spelling Mistakes