Starting Your Mac with Boot Option Keys
Part of the Switching to a Mac For Dummies Cheat Sheet
When your Mac powers up, it normally uses the system on its internal hard drive unless you change this behavior by choosing System Preferences@@→Startup Disk. You can also change what your Mac does by holding down the following keys or key combinations during startup.
Print these boot option keys, because they're particularly helpful to have when your Mac is having trouble.