When it comes to commas, the thing that people seem to struggle with most is whether or not to put a comma before the conjunctions "and," "but," and "or."  I just want you to know that knowing when to put a comma before a conjunction is not one of those mysterious skills that normal people will never grasp; I learned this in elementary school.  Like with many things, there is a simple trick to it that you were simply never shown.

When I was in high school, I once heard a student ask the English teacher about when it is appropriate to put a comma before a conjunction.  Her response?  "It's a judgement call."  Several students in the room jotted this down in their notes hastily, like she had just imparted the secrets of the universe.  WRONG.  She was so wrong, and my whole being was screeching against it on the inside.  Needless to say, they fired her two years later.

Okay, here's the trick:
A comma is appropriate and required before a conjunction if the second half of the sentence, which occurs to the right of the conjunction, makes sense when separated and placed by itself. The way I was taught was to cover the first half of the sentence (including the conjunction) and read what is left aloud to determine whether it is a fragment or a whole sentence that makes sense.  If it is a fragment, no comma. If it is a whole sentence by itself, yes comma. 

Here's an example of a sentence that uses a comma before a conjunction correctly:

That woman was supposed to be an English teacher, but she didn't know jack.

If we "cover" the first half (conjunction included)... 

That woman was supposed to be an English teacher/, but she didn't know jack.

...we get this:

She didn't know jack.

This sentence has a subject and a verb and makes sense by itself.  Therefore, a comma is appropriate.  Now let's look at a sentence that doesn't require a comma before the conjunction:

The student raised his hand and asked a question.

 And then we cover the first half and get this:

Asked a question.

Notice that the second sentence makes no sense by itself, because it is a fragment.  There is no subject performing the action.  There is not even an implied "you" performing the action, such as in the sentence, "Do your homework."  The subject of the sentence is still the student from the first half, so the second half can't exist without the first half.  Because of this, it must be attached and not separated by a comma.

Here are a couple more correct example sentences to which the trick has been applied, just in case you're still confused.:

We could go to KFC for some fried chicken, or we could go to Church's.
We could go to KFC for some fried chicken/, or we could go to Church's.
We could go to Church's. 
Complete sentence; yes comma.

NASA is funding research for a food printer and plans to print pizza first. (True, btw)
NASA is funding research for a food printer/ and plans to print pizza first.
Plans to print pizza first.
Fragment; no comma. 


I don't like Kim Kardashian, but I love her clothing line at Sears. (Also true)
I don't like Kim Kardashian/, but I love her clothing line at Sears.
I love her clothing line at Sears.
Complete sentence; yes comma.

I tried to get some work done around the house but accidentally took a nap instead.
I tried to get some work done around the house/ but accidentally took a nap instead.
Accidentally took a nap instead.
Fragment; no comma.

Now that you've got this down, go forth and use your newfound comma skills for good!  Or evil.  Just as long as you use them, really.