So I wrote a whole long post about what I though was a good photo, then realized I didn't think that way, not really. What's a good photo varies from frame to frame. No way to codify it. There are photographers and subjects that bore the hell out of me most of the time, but sometimes they break their (subjects or the way the photog looks at things) mold just enough to be really interesting. And then there are a few that always seem amazing.
To learn to look at photographs, go to a library and look through every photo book they have. Really look at the pages. When you've looked at 10,000 images (maybe 100,000) that other people think are good, you'll have a better idea of what you like, too. If you do this while doing the steps below, even better. You'll learn a lot about what makes a good photo, and how it keeps changing. That's all. It's been a long day. I'm gonna go disco nap before I go out. Not to a disco though. Those are for losers.