In a nutshell, when you add anything to an image, you should make a new layer first. This keeps every element of the image separate.
For example, I have a kickass Saving Private Ryan image of Ramelle and I want to place Tom Hanks in the foreground. Instead of just pasting Tom into the Ramelle image (essentially merging both images together) I paste Tom as a new layer. This keeps both image layers separate. So, if I wanted to remove Tom from the pic, I could just delete that particular layer.
Layers are useful for blending using transparency and a whole slew of other things. Here's a pretty simplistic tutorial on layers that may help out:
http://www.digitalhints.com/pstuts/pslayers.htm
As with any software, you have to tinker with it and play around to get good at it. Do some reading and apply what you've learned.
Good luck.
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