Use a comma to mark the boundary of a ‘preliminary’ unit at the start of a sentence.
Frequently, we begin sentences with a single word or a longer wordgroup to indicate time, place, manner, reason, or other information relevant to the main statement that is to follow:
When we are speaking, we help our listeners by signalling the boundary of such preliminary units with a change in intonation and a pause. (Try reading that sentence – and this sentence – aloud. You will hear the changes of tone and timing.) When you are writing, give your readers similar help by signalling the boundaries of preliminary units with commas.