Comma sense—a fun-damental guide to punctuation suggest to use the comma to set off introductory elements, which are reported to be:

• an adverb: First, I need to call my girlfriend.
• a prepositional phrase: After dinner, let's go to see a movie.
• an appositive: A stumbling giggler, Lumpy was hardly prepared for the relay baton suddenly being thrust upon him.
• a participial phrase
• an infinitive phrase: To be honest, I think you are over-reacting.
• a dependent clause: If you are going to leave this room, I am not going to call you anymore.

This is an example used by that guide to show what happens if you don't use the comma after an introductory element:

After retiring my wife, my parents, the kids, and I plan to travel around the country.

Comma sense—a fun-damental guide to punctuation suggest to use the comma to set off introductory elements, which are reported to be:

• an adverb: First, I need to call my girlfriend.
• a prepositional phrase: After dinner, let's go to see a movie.
• an appositive: A stumbling giggler, Lumpy was hardly prepared for the relay baton suddenly being thrust upon him.
• a participial phrase
• an infinitive phrase: To be honest, I think you are over-reacting.
• a dependent clause: If you are going to leave this room, I am not going to call you anymore.

This is an example used by that guide to show what happens if you don't use the comma after an introductory element:

After retiring my wife, my parents, the kids, and I plan to travel around the country.