There are only two instances in which you should place a comma before "and":
Place a comma before "and" if it joins two complete sentences.
Ex. Tom and Betty went to the store, and later they went to the beach.
Place a comma before "and" when it precedes the last item in a series. (Remember that to have a series, you must have a list of at least three items).
Ex. Tom and Betty bought milk, eggs, and cheese at the store.
Note: Some people consider this use of the comma unnecessary. If you do not know what your instructor prefers, I recommend using the comma until told to do otherwise.
