1. Use comma when there's an identifier

Identifier are words that uniquely describe the noun, before the noun.

Wrong: I went to see the movie, Avengers with my friend, Sarah.

Correct: I went to see the movie Avengers with my friend Sarah.

Correct: I went to see the Robert Downey Junior's latest movie, Avengers with my best friend, Sarah.

In the first example, you don't need a comma before Avengers and Sarah as there are more than one movie (other than Avengers), and you likely have more than one friend, Sarah.

In the third example, there's an identifier "Robert Downey Junior's latest" that uniquely describe the movie, and "best friend" Sarah. When the noun is unique, the comma is needed.

The general rule is if the noun is not the only thing in the world described by the identifier, leave out the comma. But if the identifier describe that noun and that noun alone, the comma is required.

2. Use a comma before and after the noun

Use a comma if the identifier is preceded by a, an, a number, or any quantifying phrase like a couple of.

For example:

A local merchant, Bob Hamilton, has opened his second Taco Bell.

Two members of the Hall of Fame, Johnny Bench and Willie Mays, will give speeches at the event.

3. Don’t use a comma If nothing comes before the identification

It means the word the is implied.

The artist David Michelle is a master of caligraphy.

I love the Pianist Masamori.

Even though David Michelle and Masamori are unique, the comma is not needed since the word the before them is already implied.

4. Commas after Yet, But, And are not needed.

Wrong: But, I don't agree.

Correct: But I don't agree.

Wrong: And, this book became very popular.

Correct: And this book became very popular.