How to make a single-player world in minecraft multi player?

Okay so I was showing my friend my world in minecraft and he wants to play on it, the thing is that I don't know how. Can someone please explain this to me? And if I have to buy a server how much does it costs and where can I go to buy it?

You could give them the seed# and then they could play it by themselves.

I don't know if you can but you can try to copy the world somehow and make it multiplayer

1. Get Evolve.
2. You and your friend(s) get Evolve account.
3. Get into the same room in Evolve
4. Whoever has the map in SP log into it IN SINGLEPLAYER.
5. Go to the menu and click "Open to LAN"
6. The one(s) who want to join go to the multiplayer menu.
7. It will be there under "Local Servers"