I am noob so that's why that images are big. Don't blame me please.
1) Start a new single-player game in Minecraft
2) Once the game has started, press Esc to open the menu.
3) Click "Open to LAN".
4) Click the "Mode" button to select the game mode for other players. You can select Creative, Adventure, Survival or Spectator
5) Click the "Allow Cheats" button to allow other players to use cheats (commands).
6) Click "Start LAN World".
7) Note the address. You will see the message Local game hosted on XXX.XXXX.XXX.XXXX:XXXX". Copy the address to give to others.
8) Start Minecraft on the other computer(s). These computers must be connected to the same network as the first computer.
9) Click "Multiplayer".
10) Select your game and click "Join Server".
11) Add the server manually if you can't find it. Click "Add server", and then enter the address that was displayed earlier.
Have fun :D