Often, the boot order is set by default to look in the optical drive (CD/DVD) for a startup disk. But, if not... or, if installing from a USB flash drive or external drive; you'll need to get into the BIOS to change the boot order.
Most BIOS' don't react to mouse movements - you'll usually only be using the keyboard to move through menus, make changes and save.
The POST (Power On Self Test) screen should let you know which key, or key combination, to press to enter the BIOS. You'll see this screen within the first few seconds of booting up (starting), or re-booting (restarting) the computer. Many times entering the BIOS will be called entering the Configuration, or entering the Setup.
If all else fails; try booting up while the keyboard is unplugged to purposely get an error that may allow you to see the key(s) necessary to enter the BIOS. Sometimes access won't be granted while using a USB keyboard. In that case, connect a PS/2 keyboard and try again.
Note: If you are asked for a password you don't know; open the computer case and carefully remove the "BIOS battery" (it looks like a round watch battery) for 5 seconds or so, snap it back into place and try again.
See How to Access the BIOS Setup Utility and Change the Boot Order from About.com. Learn much more about the BIOS at the Basic Computer Operation Tutorial BIOS page.