Use a colon (:)
1) When we give an example
There's only one book for me: Twilight.
2) When we are about to mention a list
You need to bring the following: pencil, eraser, ballpen, and paper.
- A colon is NOT used if a verb comes before the list.
The requirements are pencils, erasers, ballpens, and paper.
- A colon is NOT also used if a preposition comes before the list.
I want to go to Singapore, Malaysia, and Thailand.
3) Before subtitles of books, articles, chapters, etc.
The name of the computer game she likes to play is Sims 2: Double Deluxe.
4) When you express time
It's 8:00 a.m.
5) In the greeting part of a formal letter or business letter
Dear Mr. Juan:
6) In literary references between volume and page or between chapter and verse
7) When it comes to ratios
The wheel cleaner was diluted 3:1.
8) To indicate dialogue
Dr. Heinz Doofenshmirtz: Then no one can stop the wrath of Dr Heinz Doofenshmirtz!
Vanessa: Doctor? Since when are you a doctor?
Dr. Heinz Doofenshmirtz: [Shows her his certificate] They don't just give these to anybody, you know.
Vanessa: [Looks at certificate's price tag] Anybody with $15, they do...
Dr. Heinz Doofenshmirtz: [Snatches certificate from her] OK, that's enough looking.
9) Before a long, formal quotation
John Lennon once said: "Song writing is about getting the demon out of me. It's like being possessed. You try to go to sleep, but the song won't let you. So you have to get up and make it into something, and then you're allowed to sleep. It's always in the middle of the night, or you're half-awake or tired, when your critical faculties are switched off. So letting go is what the whole game is."
10) After words like caution, wanted, or note
Here's a video from eHow also discussing when to use a colon.
Attribution to Rebecca Elliot and her book Painless Grammar and to EHow
Other punctuation marks:
Parentheses, Brackets, or Dashes