Hold the relevant key/s before powering on.
Startup Manager: Option (⌥) Set boot source as default: Get to Startup manager then hold Control while selecting the disk Boot from CD/DVD/USB: C Target Disk Mode: T Check for Option ROM firmware: Option-Shift-Command-Period
Safe Mode: Shift (⇧)
Apple Hardware Test: D Apple Hardware Test over the Internet: Option (⌥)—D
Install the latest macOS that was installed on your Mac: Command (⌘)-R Upgrade to the latest macOS compatible with your Mac: Option-⌘-R Install the macOS that came with your Mac, or the closest version still available: Shift-Option-⌘-R
Single-User mode: ⌘-S Verbose: ⌘-V Boot from NetBoot server: N Boot from NetBoot server using the default image: Option (⌥)—N
Reset PRAM/NVRAM: Command (⌘)—Option (⌥)—P—R, then release the keys after you hear the startup sound for a second time Reset SMC: Shut down your Mac, and then press and hold the left-side Shift (⇧)—Control (⌃)—Option keys along with the Power (⎋) button, all at the same time, until the computer turns on. Now release the keys and then turn your Mac back on using the power button.
Eject removable media: Media Eject (⏏), F12, mouse button or trackpad button
OS X Startup Volume: X (will start up your Mac from an OS X startup volume when it’d otherwise boot from a non-OS X volume, like a Windows partition)
Prevent Login Items from opening: To override your current settings and prevent Login Items and Finder windows from your last session from opening when you log in, hold down the Shift (⇧) key after clicking the Log In button in OS X’s login window until the desktop appears. Prevent automatic login: If you enabled the option to automatically log a specific OS X account every time your Mac starts up, you can override this setting by holding the left Shift (⇧) key after OS X’s boot progress indicator appears.