Commas are used after introductory words, phrases, and clauses.

Silently we clutched each other's hands as we awaited the doctor's verdict.

Afterwards we joyously informed our families of my pregnancy.

After an introductory phrase

Slipping and sliding in the long forgotten pools of stale beer I was quickly reminded of our night of debauchery and my painful hangover.

Trying to maintain some small remnant of dignity my father quickly loaded the long-collected contents of his desk and walked quietly out of what had been until that moment "his" office.

If a short introductory phrase is clearly understood and does not cause confusion for the reader, the comma may not be necessary.

At that moment I realized my father was just as vulnerable as I.

After introductory clauses

Whenever I think of the O.J. trial I will always remember the dramatic conflicting reactions of my classmates.

As the foreperson of the jury announced their decision each student roared with either joy or anger about the verdict.