Help, first Audi problems
 

Just purchased a 2003 Audi A4 Quattro. Went to fill it up and only got 8 gallons before gas started pouring out the bottom until there was a puddle as big as the car. What could have caused this? After I stopped pumping I was left with half a tank. A check engine light came up, went to autozone and they told me that the Pistons were misfiring, the worker assumed this was because of lack of fuel pressure as a gas smell was strong.

This is going to sound trite, but you have a serious leak. Clearly something to do with the tank, and not the fuel system as I assume the car was shut off while you were filling it. This is not one I've heard of, I suspect not a common issue, you are going to have to look, or have someone else look. Should be easy to find, although being easy to find may not mean it is cheap to fix. Anyway, I don't think you should drive the car, it sounds like a fire is a very real possibility. By the way a 2003 is a B6, this is the B5 forum, for future reference you will get better answers there.

Check the filler neck, and the return hose connection, fuel filter etc.
I would put the car up on a ramp or hoist and with a flashlight start tracing the lines. Lots of connections and rubber fuel hose that can split after time of impact.
The misfire codes are probably lack of fuel pressure as its open somewhere.