Use an exclamation point to indicate strong emotion (happiness, anger, disgust, astonishment, and so forth), commands, or forceful language.
- Yuck!
- Come here!
- They all went shopping, and they forgot to invite me!
It is ungrammatical to use more than one exclamation point.
Right:
It was the most amazing thing I ever saw!
Wrong:
It was the most amazing thing I ever saw!!!