Basics are:

• Saving

  1.Decide what exact information u want to save

  2.Make a script that gathers this information in data array

  3.Make save menu(and pause game)

  4.Save this data array as txt file(for starting) on "Save button pressed". Better use binary files.

  5.Return to game.

• Loading

  Make loading menu and loading screen.

  1. Let player choose which file to load
  2. Open your save file for input.
  3. Make a script that inputs data from your file to data array. Then it accepts specific data to specific
     objects on your scene. Of course you need to instantiate mostly all scene with all objects with properties from data array.
  4. Start game on loaded scene.

It is rather big code and it depends on what you want to save. You can read about import/export and files in reference documentation.