As you know, photoshop is an amazingly complex and powerful tool, and as such there are several ways to select just part of an image (the shoe) and remove the unwanted areas (the background). I would suspect that for an object with relatively clean edges (like a shoe) your best bet would be to create a path around the shoe, turn that into a selection, selecting inverse, and then deleting the background.
That's the 10 second answer. If you do a google search on the topic you'll find lots of photoshop "how-to" type sites. Photoshop is one of those amazing tools that no matter how much you use it you're always finding new tricks and powers... My advice is just to play around and practice, and soon enough you'll be getting better and better results.
Have fun!
Rich