Writers often confuse the colon with the semicolon, but their uses are entirely different.
When to Use a Colon
The colon focuses the reader’s attention on what is to follow, and as a result, you should use it to introduce a list, a summation, or an idea that somehow completes the introductory idea. You may use the colon in this way, however, only after an independent clause:
He visited three cities during his stay in the Maritimes: Halifax, Saint John and Moncton.
Their lobbying efforts were ultimately useless: the bill was soundly defeated.
My mother gave me one good piece of advice: to avoid wasting time and energy worrying about things I cannot change.
When Not to Use a Colon
You should not place a colon between a verb and its object or subject complement, or between a preposition and its object:
[WRONG] His neighbour lent him: a pup-tent, a wooden canoe, and a slightly battered Coleman stove. (colon between verb and objects)
[RIGHT] His neighbour lent him a pup-tent, a wooden canoe, and a slightly battered Coleman stove.
[WRONG] Her three goals are: to improve her public speaking skills, to increase her self-confidence and to sharpen her sales techniques. (colon between verb and subject complement)
[RIGHT] Her three goals are to improve her public speaking skills, to increase her self-confidence and to sharpen her sales techniques.
[WRONG] We travelled to: London, Wales and Scotland. (colon between preposition and objects)
[RIGHT] We travelled to London, Wales and Scotland.