First you must have the NATIONAL DEX.
Then Follow the steps below:

1) Catch a ZUBAT.
2) Train it until it evolves into a GOLBAT.
3) Train your GOLBAT so it will like you.
4) Train it more until it evolves into CROBAT.

ZUBAT - Level. 22 -- GOLBAT - (you must have NATIONAL DEX) Happiness – CROBAT.