If you want to upload the world files: take the world folder in the "worlds/" folder of your Minetest installation folder, then compress it (.zip format is preferred).
Then upload it to a website like
omploader.
If you want to start a"built-in" server with a specific world: go to the Advanced tab of the Minetest main menu, select your world, put no address and port 30000, start the server. You will have to forward ports if you want people from the Internet to connect.