A comma usually follows an introductory word, expression, phrase, or clause.

Some writers omit the comma if the introductory element is short and does not seem to require a pause after it.

At the racetrack Henry lost nearly his entire paycheck.

However, you will seldom be wrong if you use a comma after an introductory element. If the introductory element is followed by inverted word order, with the verb preceding the subject, do not use a comma unless misreading might occur.