Not much actually. As soon as you can play some decent eighth and sixteenth notes with your hands and feet, start feeding yourself with plenty of quality repertoire.
Sight-read and improvise daily. Make sight-reading and improvisation your mantra.
Not only you will find this immensely satisfying but very soon you will discover that it becomes easier and easier to play unfamiliar organ music.
And as soon as you can master just a few pieces and are able to create some short musical ideas on the spot, go out of your way to schedule some public performance opportunities.
Maybe a church service here and there. Maybe a joint recital. You know, low risk, big smiles.
There are too many people who practice and practice dry exercises for months, even years all with the hope of being more secure, more ready.
You see, we will never feel safe in front of strangers so we might just as well jump in and get over that fear.
There will be plenty of failures. Plenty of self-doubt. Who am I to play in public in front of a (paying) audience? Do I really need this?
There will be plenty of successes, too. Perhaps a tear on a listener's eye. Perhaps some kid who will want to play an organ. Just like somebody inspired you.
Don't hide behind exercises and practice rooms.
Inspire others. We need your art. Now more than ever.