I have found and strongly recommend, that to work your way through the photoshop tools by having some fun!
Take pictures of your friends and try different things, if it works out or not doesn't matter, you will have learnt loads, which you can take to your "proper" pictures.
This image came about, as the faces you see are my cousins, who have an unnatural obsession, with the computer game, Final Fantasy 7. Yeah ok I have completed it twice but thats beside the point....
To insert the faces I was learning all about using layer masks, blend options and layer styles, to make all the separate objects to fit in.
Here I read in a magazine about making people look like robots and thought what a cool idea. Plus something great to learn. For this effect, you learn about the clone stamp, as you have to remove the eye brows and features, colour changing a picture and using the photoshop pre-set effects like the plastic wrap filter.
So are these images any good? thats not the point is my answer.
you learn a lot from having a play,
When I first started on Photoshop, a simple cut out would take me an age! But now you look at it different and your work flow is a million times faster, but the bottom line.... it is FUN!