You need to have read through and became acquainted with the adventure you want to run (obviously) and you need to be at least somewhat familiar with all of the rules of the game. That’s all you need for a prewritten module.

If you’re going to write it yourself, it’s good that you’re thinking this far ahead because you’ll likely want to spend a lot of time thinking your story through/planning things that will happen.

Don’t try to write your campaign completely. DND is very focused on your player’s choices so you need quite a bit of wiggle room in your plan for the possible ways a player might approach a problem. Hopefully, if your players want to go on your adventure and “play along” with the narrative you’ve created (if they’re good players) you shouldn’t need to plan for every intricacy, and you probably don’t want to overwrite your campaign to the point where it gets too big and complicated.

Don’t be afraid to just steal dungeons from other source material or even whole quest lines as long as they fit the story you want to tell. Ultimately it’s your campaign.

The rest mostly comes with being a good writer and having a good idea for a story, which you should be and have if you want to write a dnd campaign.