There are many resources online that will help you decide whether to use a comma.

As a general rule, a comma is used when there is pause in a sentence. If the sentence is hard to read without a comma, use one. (Note the two commas in the previous two sentences.

There is a trend towards omitting commas after short introductory phrases at the beginning of a sentence.

Example:

In this sentence a comma is optional. Note that the sentence is easy enough to read.
(compare)
In this sentence, a comma is optional.

Please go to one of the online grammar sites for specific rules about comma usage.

When in doubt, read the sentence out loud. Readability is probably more important that any hard and fast rule, unless you are submitting an English dissertation.

© Miller McMillan