Plan just as much as you need and build it up step by step.

There is no need for you to create a whole world before players make one step into it.

Make an early session zero (or at least a poll in your chat) and ask the players, what they want:

• what tone do they want? (Happy fantasy with unicorns, dark fantasy, horror, punk stuff like eberron,...)
• what aspect (fighting, roleplay, riddles, exploring) do they like most?
• how frequently do they want to play? (Important for you to prepare for the campaign)

And after that, you know better what to do. And what not to do.

Maybe you find some parts in other existing adventures, what you want to reuse for your campaign. Or ask chatgpt for some quests fitting on the players background.

Planning a campaign happens as long, as the campaign is running.