Out of the box, the Apple Magic Mouse has a smooth surface that hides its two button nature. On top of that, the default in System Preferences is a one button mouse. For those who prefer a classic two button mouse, here's how to fix that.
1. Under the Apple Menu, select System Preferences.
2. In System Preferences, click on the Mouse icon.
3. In the mouse preferences, check the box "Secondary click" and select the popup for "right". That makes the right side of the Magic Mouse act as a right mouse button. That's used in the Finder for contextual items and by apps for other functions.
If you don't want to have a two button mouse all the time, you can always leave the secondary click checkbox unchecked and access the right button, as needed, by holding down the Control Key when you left click.
The option to make the secondary click the left button is, of course, for people who are left handed.