Press keys immediately after you hear the startup tone

C Boot from a CD or DVD (such as the Mac OS X Install disc)
Select a volume to boot from
X Force system to boot into OS X
N Attempt to start up from a compatible network server (NetBoot)
Option-N Start from a NetBoot server using the default boot image
D A Start Apple Hardware Test (AHT)
Option-C-P-R-B Reset non-volatile RAM (NVRAM)
Shift Start up in Safe Boot mode and temporarily disable login items
C-V Start up in Verbose mode
C-S Start up in Single-User mode
T Start up in FireWire Target Disk mode
Eject, F12, or mouse button Eject any removable media, such as an optical disc