How to make a singleplayer world into a server?

I'm trying to turn my hardcore singleplayer Minecraft world into a server so my friend and I can play on it without using LAN. Is there a way to do this?

Use Hamashi, far people online will join your LAN game like it's a server

Yeah, you just copy your singleplayer world into the world file for the multiplayer server tool's world folder (it generates one for you during start up). Then to play with someone you just use Hamachi, which is by far the easiest way to host a server I've seen.