Adverbial clauses and phrases that begin a sentence are usually set off with commas.

• Having made that decision, we turned our attention to other matters.
• In order to receive a high school diploma, a student must earn 16 credits from public or private secondary schools.
  
• In addition, staff members respond to queries, take new orders, and initiate billing.

If the sentence can be easily read without a comma, the comma may be omitted. The phrase will usually be short—four words or less—but even after a longer phrase the comma is often omitted.

• As cars age, they depreciate.
  

or

• As cars age they depreciate.
  

• In January the firm will introduce a new line of investigative services.
  
• On the map the town appeared as a small dot in the midst of vast emptiness.
  
• If nobody comes forward by Friday I will have to take further steps.