Switching from Mac OS 9 to Mac OS X
Step 1: Open the Startup Disk control panel. You see your hard disk volume(s) in a list that also includes Network Disk as shown here. You can ignore Network Disk.
Startup Disk control panel with several hard disk volumes
The current startup volume is highlighted. In this example, it's the disk named Brendan. If different versions of Mac OS are available on different hard disk volumes, they are noted in the Version column. In many cases, both Mac OS 9 and Mac OS X will be installed on the same volume.
Step 2: Click the disclosure triangle next to a volume to see what system software it contains. More than one startup choice is listed on the volume "main drive," where both Mac OS 9 and Mac OS X are installed.
Two system software installations on "main drive"
Step 3: From the list, select the Mac OS X System you want to use as the startup disk.
Step 4: Click Restart.
If Mac OS X does not start up, you can do some basic troubleshooting.
Switching from Mac OS X to Mac OS 9
Step 1: From the Apple menu, choose System Preferences.
Step 2: From the View menu, choose Startup Disk. The Startup Disk pane appears, and the current startup System Folder is highlighted. The text below the slider also notes the selection, as shown here.
Mac OS X 10.1.2 on main drive selected
Step 3: Look at the lock button. If the lock appears closed, click the button to make changes. Enter an administrator account name and password, then click OK.
Step 4: Click the System Folder icon for Mac OS 9 disk you want to use as the startup disk.
When many choice are available, drag the slider to see additional choices. A gold "9" badge appears on the icon of a Mac OS 9 System Folder if it is located on the same volume as Mac OS X.
Mac OS 9.2.2 on main drive selected
Step 5: For Mac OS X 10.1 and later, click the Restart button. A confirmation dialog appears. Click Save and Restart. In versions of Mac OS X prior to 10.1, the restart button does not appear here. Instead, choose Restart from the Apple menu.
Confirmation dialog for change of startup disk
See this article for more information