Okay, so even I was confused for a while about this one. Some people say you have to have a comma before all conjunctions (and, but, or, etc.), and others say you don't ever need a comma before 'and'. So, which is it?
There are two main situations when you'll need a comma before 'and'. First, is called a serial comma (by the way, I love that name! It makes me imagine this killer comma! ... literally.) The serial comma is used when you have three or more items in a list.
Example: In order for humans to live they need food, oxygen, and love.
Situation number two: You use a comma when you have two independent clauses (aka: something that could stand alone as a separate sentence if it wanted.)
Example: Tara is really devoted to her studies, and she passes every test.
When you won't need a comma: Tara is a really devoted to her studies and passes every test.
'Passes every test' cannot stand alone, but 'she passes every test' can. See the difference?