Photoshop is a great tool to make art (if you know how to use it). Some of the best ways to get to know what Photoshop can do is to see examples. Luckily, there are HUNDREDS of sites with great step-by-step tutorials on how you can edit photographs. Here are a couple to check out:
http://photoshoptutorials.ws/http://psd.tutsplus.com/
http://www.tutorialized.com/tutorials/Photoshop/1
http://photoshoptutorials.ws/http://psd.tutsplus.com/
http://www.tutorialized.com/tutorials/Photoshop/1