Everyone can upload their worlds to Realms. All realms you can play have been uploaded by other builders.
To publish a world to realms follow the following steps:
When uploading a world you can give it an initial difficulty. The difficulties are rated from 1 up and including 10.
Trophies are rewarded to players on completing a realm. Trophies ratings are given manually to ensure that the competition and achievements stay fair.
Builder points are automatically rewarded to the builder of the world when a trophy rating is given to a certain world.
It could be that you uploaded your world but you want to change some minor details. For this the .update command can be used. Simply join the realms lobby and use the command to start the update process.