Use a colon before a list when the list is preceded by a complete independent clause.
Never use a colon to separate a preposition from its objects or a verb from its complements.
Some form of the word follow usually indicates a colon before the list.
Correct: John has all the ingredients: minced clams, milk, potatoes, and onions.
(The list is preceded by a complete independent clause.)Incorrect: For their anniversary they went to: Aruba St. Martin, Jamaica, and the Bahamas.
(The colon separates the preposition to from its objects.)Correct: For their anniversary they went to Aruba St. Martin, Jamaica, and the Bahamas.
(No colon needed)Incorrect: To make clam chowder you need: minced clams, milk, potatoes, and onions.
(The colon separates the verb need from its complements.)Correct: To make clam chowder you need minced clams, milk, potatoes, and onions.
(No colon needed)
Either incorrect sentence above could also be corrected by adding a form of the verb follow.
Correct: For their anniversary they went to the following places: Aruba, St. Martin, Jamaica, and the Bahamas.
(Now the word places is the object of the preposition to, and the colon follows a complete independent clause.)Correct: To make clam chowder you need the following: minced clams, milk, potatoes, and onions.
(Now the following is the object of the the verb, and the list follows a complete clause.)