After selecting part of an image with either the Marquee, Lasso, or Magic Wand (or any combination thereof), you'll probably want to copy the selection and paste it into another image. Here's what you need to do to copy and paste:

• Once you've defined a selection and are happy with it, be careful not to click anywhere on the image (if you do, you'll lose your selection!).
• Copy it by choosing Edit>Copy.
• To paste the copied selection into another file, open that file and choose Edit>Paste.
• The selection will appear in a new layer of its own.