The key to success in Photoshop photo manipulation is to preserve editability. All the changes you make to the original photo must be non-destructive. This allows you to turn your changes on and off. Another part of doing non-destructive work is to keep a back-up of your work. Work on a copy of your original scan. Also, as you work, it is a good idea to save copies of your file as you progress. To do so, simply go "Save As" and choose a new name with a number one, then two and so on. You can also go "File >Save a Copy" and choose a new name with a version number.
Please see the podcasts on the left side of this page for all the software knowledge you need to complete this assignment. You need to execute the repairs as I do them in the videos to earn a high grade.
Avoid cloning large areas of your photo like a wall or a sky. Restrict your cloning/healing to spots, tears and cracks. If you need to repair a large surface, use the Patch Tool.