If the picture I need to repair doesn't have enough background to copy, or doesn't have a good background, what should I do?
Remember that Photoshop allows you to have more than one file open at a time. You can borrow from another picture and copy the selection onto a new layer of the picture that needs fixing. Shrink it or enlarge it so that the texture is in scale with the rest of the scene, and then copy and paste it as much as needed. If you have a digital camera, shoot lots of backgrounds and keep them in a special file on your computer or on a zip drive. Then when you have a problem photo, you have ready-made scenery to drop the subject into.
What color mode should I be working in: CMYK Color, RGB Color, Indexed Color, Lab Color, or Grayscale?
If the image is grayscale, such as a black-and-white photo, and is going to remain grayscale, stay in that mode. If the image is intended for the Web or an inkjet printer, stick with RGB. If the image will be sent to a commercial print shop for four-color process printing, convert to CMYK after you've done your retouching.