Here's why:
* Comma: The comma separates the name "Sam" from the pronoun "it," creating a pause and indicating that "Sam" is being addressed directly.
However, without context, it's impossible to tell what the complete sentence should be. Here are some examples:
* "Sam, it's time to go."
* "Sam, it's raining outside."
* "Sam, it's your turn to play."
Let me know if you have more context, and I can help you punctuate the complete sentence!