I want to echo the suggestion to just play with it. I'm *not* a photoshop professional and have never been trained in it. However, I have been able to learn quite a bit through trial and error. My stuff's not *great* by any measure, but I have been able to generate these, which I like :-)
All the blast effects are done with layers and layer effects. The backgrounds are all simple landscape shots, and my figures are cut out of very ordinary desktop shots, and other items are also cut out of product stills and the like.
Remember to adjust the levels to match up the various elements of the picture (i.e. each layer as pasted), and you may have to work with the color balance to get things to look more like they go together. The orange rock in two of the pics above, for example, looked even more out of place before I started tweaing. Some things just won't fit in, it seems, so remember to use good base photos.
You're talking about learning to use a *very* powerful graphics tool, and just like all things, you won't be very good for a while (even if you take a course, IMO). The cool thing is that when you learn how to accomplish something, you won't forget it, as you'll have learned what NOT to do about three dozen times before you get it just right.
Best of luck, and I can't wait to see your PN work.