To select an area of an image in Photoshop, the most basic option is the marquee tool. However, there's a little more to it than simply clicking and dragging.
Photoshop guru Helen Bradley runs over ways to enhance your marquee selections. Much of this material will be familiar to Photoshop experts, but it's useful information for newcomers, even on the basics like why the circle selection tools are so strange:
A circle or ellipse is drawn as if it were placed inside a square or rectangle shape so you start drawing the shape from a corner of its square or rectangular container. All this makes it very hard to position a shape accurately. To make things easier you can draw your shapes from the center outwards by holding the Alt key (Option on the Mac), as you drag on the marquee tool.
For more Photoshop hints, check out how to improve your cropping technique.