Found this by searching for startup disk in the help menu...
Changing your startup disk
You can make your computer start up from a CD, a network volume, a different disk or disk partition, or another operating system (such as Mac OS 9 if your computer supports it). To do so, you change your startup disk.
To change your startup disk:
Open System Preferences and click Startup Disk.
If necessary, click the lock icon and type the name and password for an administrator user.
Click the icon of the system folder or system volume that you want to use, and then click Restart.
WARNING: When selecting a network startup volume, make sure you select a valid network startup volume and not a network install image. Choosing a network install image reinstalls your system software and may erase the contents of your hard disk. A standard network volume icon appears as a globe and Mac OS X system folder. When you select a network volume icon, a message appears in the Startup pane describing the volume. A network install icon appears as a globe with downward-pointing green arrow.
If you don’t see a disk that you expected to see, that disk may be able to start up some computers but not your computer. For example, a disk that can start up an Intel-based Mac may not be able to start up a PowerPC-based Mac.
If your computer is on a network and a network startup volume is available, you can start up from the network startup volume by holding down the N key as you restart your computer.
If you start up your computer in Mac OS 9, use the Startup Disk control panel in the Apple menu to switch back to your Mac OS X startup disk. Then restart the computer.
Another way to select a startup disk is to hold down the Option key as you restart your computer. You’ll see the available startup disks and can select one.
Regarding the mouse:
http://support.apple.com/kb/HT1537
