There are three way you can learn how to play.
1) Tutorial level :- There is a tutorial level that can be played from contract board if you want (NTR warning since some find it offensive).
2) Basement Tutorial :- You will understand basic tool usage and ghost capturing.
3) How To Play option :- If you press 'Escape' you will see this option in the menu.
You can also watch this video on how to detect a ghost.