Place a comma after most introductory words and phrases:
Moreover, we managed to revive the ferret.
In the meantime, let’s just kick back and listen to some Zeppelin.
If everything goes as planned, José will have the best birthday party of his life.
The introductory comma is optional for single words and very short phrases (three words or less):
Sometimes I feel like the north end of a south-bound mule.
Last night Lulu found $1.36 in change underneath the couch cushions.
But even very short phrases or single words sometimes need a comma to ensure clarity:
For now, we must assume the worst.
Outside, the town went on about its business.
That said, I think you have a great personality.
Omitting an introductory comma can seriously (and comically) confuse the reader:
While you were eating Rahul called about the Fountains of Wayne tickets.
Just when Tina was ready to leave her husband called from Poughkeepsie.
Without a comma after eating, the first sentence seems to be about cannibalism.
Without a comma after leave, the second sentence seems to be about an impending divorce.