Some people misuse the colon, so be careful. If a colon could have one wish, what would it be?
That people understood its real purpose.
Use a colon to amplify or illustrate something about the first clause: that, as you can see, is its true purpose.
Don't use a colon to simply separate two clauses: usually its cousin, the semicolon, makes more sense to use in those cases. (You see, the colon does not work as well as a semicolon would. The clauses are not similar enough.)
A colon is also useful when introducing a list: this is its most popular, familiar, and common use.
Here's a more advanced tip:
The colon likes to be sneaky. It often asserts itself too soon. It must only come after a grammatically complete sentence.
This colon includes: informal, unhelpful, and incorrect usage. (Our nosy little friend cut off the first sentence.)
This colon includes the following: formal, correct, and helpful usage. (There you go, little colon! Patience. Wait until the first sentence finishes its thought).
So, make sense?