How do I get started mining bitcoin with a GPU? I'm totally new to this, but love this idea!

I want to help, (and generate coin), and have only installed Bitcoin-Qt version 0.8.1

It ran, synchronized; I encrypted my wallet.

But now what?  Will it use my 2 NVIDIA gtx580 GPU's on its own? Do I need to do anything else, other than just let it run?  

Got the green checkmark; up to date
I have 8 active connections and my wallet is both locked(?) and encrypted.
I just did this today...

I found a mining pool (I picked Eligius) from here https://en.bitcoin.it/wiki/Comparison_of_mining_pools

The pools website had further instructions. In my case, I installed a mining client "bfgminer" and followed the instructions to add my ID. It sounds like for a windows OS, GUIMiner might be a good start too.

gui miner thread https://bitcointalk.org/?topic=3878.0

The key thing to realize is that there is a difference being a BitCoin "client", which is what you setup above, and a BitCoin "miner" which requires the steps I in this response.