Before we dive into specifics about the Photoshop interface, please take the time to review the consistencies in the Creative Suite Interface .
Here are some details and review specific to Photoshop:
Once you have a handle on navigating the interface, it is time to learn how to create new projects and open existing ones. The following video gives a great demonstration of how to do both of these tasks.
Tip: If the units are not what you want then input your desired dimensions followed by the appropriate unit abbreviation (e.g. 15 cm), and the dialog window will make the conversion for you.
Tip: If you want to open files more rapidly on a Mac, you can always drag the file you are attempting to open on top of the corresponding icon on the dock.