What version of Mac OS are you running? Classic? OS X? If it's OS X, which exact version (10.0.x, 10.1.x, 10.2.x, 10.3.x, 10.4.x)? Go to the Apple menu on you computer and select "About this Mac" (or "About this Computer" depending what version of OS X you're using).
Have you tried holding down the Option key after you hear the startup chime? Keep it held down until you see a selection for which drive to boot from.