If you need to boot to a secondary hard drive on your Mac, you can do so using several different means. For most PC systems, you will need to press F12, or another F-key, or perhaps the Escape or Delete keys when booting your system in order to select an alternative boot drive. To do this on a Apple computers, you use the Option key; however, there may be some constraints.
Generally you can start up or reboot your Mac, and simply hold the Option key once the screen goes black so that when it reboots it will show you the boot menu. However, if you do this with a wireless keyboard you may not see the menu and instead the system boot normally. This is because holding down the key before the Bluetooth keyboard on your Mac initializes will prevent it from recognizing the key as pressed. Bluetooth controllers are initialized once the boot chimes sound; to ensure your Mac goes to the boot menu, press and hold the Option key immediately after hearing the boot chimes, not before. This same approach goes for other boot modes as well (Safe Mode, Single User mode, Verbose mode, etc.).
When you have invoked the boot menu properly, a gray screen will display that shows available boot volumes. For systems running OS X 10.7 or later, you should see the default Macintosh HD partition, along with a Recovery HD volume; for systems running 10.6 or earlier, you should see only the main boot volume, unless you have multiple valid operating systems installed.
At this point, you can attach external hard drives, flash drives, or optical disks that contain valid operating systems, and when recognized they should appear alongside the current boot options.
You can then boot to the desired disk by selecting it with your mouse and clicking the arrow button, or by using the arrow keys to navigate the menu, pressing Enter to select the desired volume.
Any Mac that shipped with an optical drive can boot to a disc in that drive by holding the "C" key at startup. While most recent Macs supporting this feature have shipped with DVD drives, the C stands for CD-ROM, as it was implemented when Macs came with only CD drives. For those without optical drives, you can use an external USB DVD drive to insert a boot DVD and have it appear on the standard boot menu for access.
For systems configured with OS X 10.7, 10.8, or 10.9, you can boot directly to the recovery drive by holding Command-R. Additionally, most systems shipped after 2010 support Internet Recovery, which can be invoked by holding Option-Command-R. Keep in mind an Internet connection will be required for this recovery, which will download an approximately 650MB image file from Apple.
Note that selecting an alternative boot disk from the default one using this method will only be set for the current boot session. Restarting the system will revert back to the default boot disk. To change this, you will need to use the Startup Disk settings that are available either in System Preferences, in Apple's Boot Camp drivers for Windows, or in the Recovery partition.