It depends! Wait–not what you wanted to hear? Sorry. Okay, let me try to break it down 🙂
If you have an introductory word (or two) that is being used as an adverb (usually answers the question of when, where, why, or to what degree), then no comma. If it’s being used as a conjunction, then go ahead and use a comma.
Here are a couple of examples:
“Someday, I’d like to go on a cruise.” — This is incorrect. Someday is functioning as an adverbial phrase, answering the question of WHEN I would like to go on a cruise. Correct would be this:
“Someday I’d like to go on a cruise.”
Other correct examples:
Tomorrow I’m going to get my hair done.
Yesterday I paid the car note.
Next week is finals week.
Now used as both an adverb (first example) and a conjunction (second example):
Now I need to pull my pie out of the oven.
Now, I know it’s a bad idea, but I’m going to do it anyway.
As always, hope this helps, and feel free to discuss or ask me if you have questions!