Commas help to organize sentences and have many uses.
They can be used in the following ways:

Dates
Commas are used to separate the parts of the date. They
are used between two numbers, or between two words.

December 12, 2006
Monday, July 4

If the date is in the middle of a sentence, a comma goes
after.
He arrived on June 10, 2007, and left three weeks later.

Lists
Commas separate items in a list.

I need bread, milk, eggs, and cheese.

Conjunctions
Commas go before a conjunction
(for, and, nor, but, or, yet, so)

Jason spent fifteen minutes doing his homework, but
it felt like an hour.
I hadn't gone to class for most of the year, so I failed
my exam.

Adjectives
Commas separate two adjectives before a noun.

I bought a new, red car.
The big, black dog is barking.

Interruptions or Non-Defining Relative Clauses
Commas are used to separate a non-defining relative
clause from the rest of the sentence. This is used to add
extra information, or make interruptions.

The man, who had never been to Paris, was very
excited.

After Introductory Words or Phrases

Use a comma after an introduction to a sentence
including times and transitional words.

On Monday, the family travelled to Florida.
While watching television, the children ate dinner
and fought over their dessert.
However, her bank account did not match her records.