Three main uses of the colon ( : )

1. Use a colon between two main clauses in cases where the second clause explains or follows from the first:
EXAMPLE: That is the secret of my extraordinary life: always do the unexpected.
EXAMPLE:  It wasn’t easy: to begin with, I had to find the right house.

2. Use a colon to introduce a list.

EXAMPLE: The price includes the following: travel to London, flight to Venice, hotel accommodation, and excursions.  

3. Use a colon before a quotation, and sometimes before direct speech.                                                                                   

EXAMPLE: The headline read: ‘Taxi Driver Battles Gangsters’.
EXAMPLE: They shouted: ‘Our families are starving! We need land!’

Reference 

Oxford Living Dictionaries. 'punctuation/colon' Retrieved from https://en.oxforddictionaries.com/punctuation/colon.