Bring your singleplayer world or downloaded map to your server.
Locate your world folder on your computer (usually in %appdata%/.minecraft/saves). Make sure the folder contains `level.dat`, `region/`, etc.
world (or whatever is set in your server.properties).If you named your folder something other than "world" (e.g., "my_epic_map"):
server.properties.level-name=world and change it to level-name=my_epic_map.