Write a complete sentence before the colon. The part of the sentence before the colon is called an independent clause. It must be able to stand alone, otherwise the colon is improperly used. To test whether you've used a colon correctly, remove the colon and what comes after it; make sure that the clause that is left is a full sentence that makes sense standing alone.
Use a colon to introduce a list or series in a sentence. This highlights the important information and adds emphasis to it.
Use a colon to set up a quotation by giving the name of the person you are quoting and hint about what the quote will say. A colon may be used when the part of the sentence preceding it gives the reader a better understanding of the quote to come. This should be an independent clause that not only introduces the quotation, but imparts some information about it.
Use a colon to make a compound sentence where the second part of the sentence fulfills an expectation created by the first part. You may choose whether or not to capitalize the first letter of the second sentence. A compound sentence is actually two sentences connected with punctuation. In the case of a colon, the first part of a compound sentence should ask a question or bring up a dilemma that is then answered or expounded upon in the second sentence. What comes after the comma makes things more clear for the reader.