AdSense

I guess I set up my blog to have AdSense - I have no idea how to use that! I see that there are ads on my blog now, but I am not seeing any money. I have received like 2 different 'gift card' looking things saying "here's $100 towards advertising your blog!" Not terribly useful for me right now.

I think I will be turning that off, as I don't see any benefit. Anybody know more about this and want to tell me if there's something I should be doing differently to get some passive income from my blog?