Open ANY single player world. Press Esc. Press Open To LAN. Set your settings. (If it is just your friend, I would suggest cheats on) You could also allow them to be on creative survival or adventure mode. After this, press start LAN world. Then the LAN world is up. For your friend to go on, he ust go to Minecraft and click on Multiplayer. There it will scan for LAN worlds. Th name of your character and world will pop up there. Get your friend to click on and it your done!