Under the selection menu you will find the color range selection tool. With an image open, go ahead and click on the color range selection tool. A small window will pop up showing your image. Now if you go ahead and roll over your actual image or the preview that has popped up you will notice you have the eyedropper tool, go ahead and click somewhere in your image. (check to make sure the selection radio button is selected beneath the preview window for this example.)
Notice the preview image showing your selection in grayscale. Now you have a few choices here for improving your selection. You can use the fuzziness slider to narrow or increase the range of color and value your selection. You can also hold down the shift key and click on another area with the eyedropper to add to your selection or hold down the alt key using the eyedropper to subtract from your selection. (I will usually use a combination of the the above options)
Other options and tips: Note the drop box that allows you to select by various colors, values and out of gamut.
Note that you can use the image itself as the preview by clicking on the image radio button and selecting one of the preview options.
Here is a very useful tip: If you want to confine your color range selection to a given area, first select an area with any of the selection tools, then open up the color range tool and select a range within that area. When you close the color range box your new section with be confined within the first selected area.
I suggest you follow the above steps and see for your self why this will make the color range selection tool another valuable tool in your selecting toolbox.