The biggest problem with commas is that for some reason (usually in grade school) teachers tell children they are used whenever you want a pause in a sentence. This is simply not true. Usually, people end up creating comma splices when they drop commas wherever they feel they need a "pause."
Although there are many rules regarding commas, modern style guides are in the "less is more" mode especially when it comes to commas. There are a few places where it is necessary to use a comma and these should be the only places beginning writers should attempt to use them:
FANBOYS: When you begin a sentence with certain words like "For," "But," "So," etc., you need a comma. When you begin a sentence with a dependent clause, (like this sentence) you also need a comma. What in the world is a dependent clause? One that is not a sentence. I am not an English teacher, so to break it down further than Internet-speak would be rather difficult. The best I can do is give you some examples:
Patricia, what are you doing?
If I go to the store, should I get some milk?
So, you think you want to be a star?
The second place you need commas is when you are using one of those FANBOYS to connect two sentences together:
I wanted to eat out, so I decided to stop at McDonalds.
You shouldn't eat out so much.
The first sentence is actually two sentences. If I removed the comma and the word "so," I could make them two sentences. However, I chose to squish them together. The second sentence is not really two sentences squished together. It is only one sentence, and the word "so" is functioning as an adverb. Therefore, you don't need and shouldn't use a comma.
The trickiest part of grammar comes when some things can go either way.
I bought a candle, a tablecloth, and some spaghetti.
I bought a candle, a tablecloth and some spaghetti.
Which is correct? Both are. When it comes to serial commas, you are delving into a gray area of grammar. In this case, if you believe the first sentence is better (like me), you are following the APA style guide (among others). If you like the second sentence, you are following Chicago style. The problem is generally not which method you choose, but the biggest problem tends to be consistency. If you choose the first example, you should ALWAYS use the comma before "and." Also, you need to make sure your editor knows you prefer serial commas.