There is a very fine "Tip of the Day" on About.com today (Monday) about using various Photoshop tools to select and cut out portions of an image.

It's loaded with information that might be helpful for several of the questions posted here.

The URL is
http://webdesign.about.com

By the time you read this, the tip will be in the "Tip of the Day" Archive.

I hope it'll be of some help.

Anne