How do i save my minecraft world?

Xbox: when you exit it'll automatically save
Computer: ^ I think it's the same thing
PE: not sure

Minecraft saves automatically every so often. But if you want to be 100% sure that it saves, when you quit the game go into the pause menu and press "save&quit". This will preform a hard save and exit the current game and return you to the title menu. When you return your game will be exactly how you left it.

You don't.

Automatically saved everytime you leave on a PC