Put a comma after introductory material.

Introductory Words
Commas set off introductory words that have no grammatical connection to the rest of the sentence.

Well, I guess I'll go eat worms.
Alex, stop acting like a clown.
Yes, I want to leave now.

Introductory Phrases
Use commas to set off introductory phrases that tell you more about the subject.

Running down the street, Robert shouted, "Wait for me!"
To get to school before the bell, Kelsey had to skip breakfast.
Looking down the street, I asked myself who could be coming.

Introductory Clauses
A clause contains a subject and a verb.

After the biker swerved, he ran into a tree.
If you run faster than any other girl, you will win the race.
Because he completed the work, Harold received a good grade.

Move on to Rule 3.