When you first set up your Minecraft server, it starts off with a freshly generated world without any built structures. However, probably you have already spent a lot of time building your own world which you'd like to play on with friends.
If that's the case you came to the right place!
How to find your single-player world on Windows 10
Press the Windows Key (Start)
Type in %appdata% and press enter.
Open the .minecraft folder and then enter the saves folder.
Find your save and drag the folder to your desktop so you could find it easily.
Rename the world so you can identify it later. You can use any name you'd like.
How to find your single-player world on Mac OS
Click on the Spotlight button in the menu bar, it looks like a magnifying glass. Or, use the Command+Space keyboard shortcut.
Paste ~/Library/Application Support/minecraft into the search.
Open the saves folder and drag the world to your desktop.
Rename the world to something that you can remember for later.
How to upload and configure your single-player world on your server
1. Log in to your Game Panel by following this article.
2. Open up your File Manager from Game Panel.
3. Delete the current world folder by using right-click and then Trash (Make sure that your server is stopped before doing this).
4. Next step is archiving or compressing your world folder so you could upload it.
4. Now drag your world archived/compressed file from your desktop to the File Manager window.
5. Once your world zip file is uploaded, you'll need to extract it by using right-click again and then Extract.
6. Now you just need to rename your world folder back to world and start your server.
Checking if the map is working correctly with the server
The last step you should take is simply log in to your server and fly around a bit to see if the map was changed correctly. If it's not changed still, make sure that your map name is using small letters only.