A colon can be used in several ways. It is important to make sure that a colon is preceded by an independent clause (a sentence that is grammatically correct on its own) when used in the context of a sentence.
Using a colon to present a list
I have four favorite colors: green, blue, red, and purple.
Using a colon to provide more information
I'll tell you what she's doing: she's stealing our muffins!
Using a colon to lead into a quotation
Liz Newman stated her thoughts clearly:
"I have never been so distraught in my life. The actions of that man are far beyond my comprehension. I hope they put him away for a long time."
Colons are typically used before quotations that are more than seven words long. Otherwise, a comma is appropriate.
Using a colon to show a subtitle
Deep Sea Diving: The Mystery Below
Using a colon in a professional introduction
Dear Dr. Pecard: