An exclamation point (!) signals strong feeling: surprise, fear, indignation, or excitement. It cannot, however, make an argument more convincing, lend force to a weak statement, or call attention to an intended irony. Use exclamation points sparingly. (Emphasis is better provided through sentence structure and word choice.)
An exclamation point is most commonly used after an interjection, phrase, clause, or sentence to indicate strong emotion or urgency.
Ouch! Go away! Stop that right now!
The exclamation point goes outside quotation marks, unless what is quoted is an exclamation. Do not use a comma or a period after an exclamation point.
I can't believe he answered "four"!
Mikie looked at her and said, "I can't believe I ate the whole thing!"
exclamatory sentences
An exclamatory sentence makes a statement so emphatically that it ends in an exclamation point. (See also sentence types.)
These prices are outrageous!
How dare you! Stop that this instant!