How to change a singleplayer world into a multiplayer world [duplicate]

I really want to know how to make a singleplayer world turn into a multiplayer world. I need to know the things needed to make it change. I found a YouTube videos about it but it didn't make much sense.

Go in to your Minecraft folder (%appdata%\.minecraft), then open the saves folder.
In there, all your Singleplayer worlds are saved.

Now select the Singleplayer world that you want to use for your Multiplayer Server, and drag it into your Minecraft Server folder.

Now, you will already have a folder inside of your Server folder called world. You'll have to delete it.
After you have deleted the world folder, you can rename the Singleplayer world folder (that you dragged into the Server folder) to "world".

Restart your server, and you're done.

Going off a more specific route than creeperkid09's answer.

You need to make a running server first before you move you're singleplayer world to the multiplayer Server. If you have already done that skip down past this next bit.....

Server Set-up

Download the newest Server file from http://www.minecraft.net/download once you have done that, move it into it's own folder (doesn't matter where the folder is as long as you remember where you put it.) and rename it to minecraft_server. Then press start and open up a new Notepad and type the following into the notepad...

java -Xmx1G -Xms200M -jar minecraft_server.exe nogui
pause

Next you save the notepad into the same folder that the Server File is in, and name it "Run.vbs" and under the name there should be a drop box, set it to "All Files" Then click Save. Next, you run the Run file. after it is done creating a few files open the EULA Notepad found in the server's folder, and change false to true press Ctrl and S to save it, then close the notepad. Then double click the Run File again. It should generate the rest of the server files then once that is done...

Server Set-up Done.

Delete the folder names "world" in the server's folder and move your singleplayer world into the server's folder and rename it to "world" all lower-case. Then run the "Run" file and the server should start up.

You will need to use Hamachi to have your friends join, or you can port-forward. But Port-Forwarding is more dangerous.

If you have any trouble with any of this let me know, I'd be happy to help you with this. I had the same problem not too long ago & I couldn't find all that much help.