Below listed are startup key combinations with for Mac computers using Intel processors. The keystrokes are different for Apple Silicon Macs.
Note
Remember, the Option key is the Alt key. The Command key is the "Cmd" key.
Press C during startup: Start up from a bootable CD, DVD, or USB thumb drive (such as OS X install media).
Press D during startup: Start up in Apple Hardware Test (http://support.apple.com/kb/HT1509) (AHT).
Press Option+Command+P+R until you hear startup sound a second time: Reset NVRAM (http://support.apple.com/kb/HT1379)
Press Option during startup: Start up in Startup Manager (http://support.apple.com/kb/HT1310), where you can select an OS X volume or network volume to start from.
Press Eject/F12, or hold the mouse or trackpad button: Ejects any removable media, such as an optical disc.
Press N during startup: Attempt to start up from a compatible network server (NetBoot).
Press T during startup: Start up in Target Disk Mode (http://support.apple.com/kb/HT1661).
Press Shift during startup: Start up in Safe Boot (http://support.apple.com/kb/HT1564) mode and temporarily disable login items.
Press Command+V during startup: Start up in Verbose (http://support.apple.com/kb/HT1492) mode.
Press Command+S during startup: Start up in Single-User (http://support.apple.com/kb/HT1492) mode.
Press Option+N during startup: Start from a NetBoot server using the default boot image.
Press Command+R during startup: Start from the OS X Recovery System (http://support.apple.com/kb/HT4718)1
1 Available on Macs that ship with OS X Lion. Some CPUs require an EFI BootROM update to support this feature (http://support.apple.com/kb/HT4904).