Your system's first boot on its own power is what electrical engineers call the “smoke test”.
If you are booting directly into Debian, and the system doesn't start up, either use your original installation boot media, or insert the custom boot floppy if you have one, and reset your system. This way, you will probably need to add some boot arguments like root=root, where root is your root partition, such as /dev/sda1.
If the machine fails to boot after completing the installation, and stops with a boot: prompt, try typing Linux followed by Enter. (The default boot configuration in quik.conf is labeled Linux). The labels defined in quik.conf will be displayed if you press the Tab key at the boot: prompt. You can also try booting back into the installer, and editing the /target/etc/quik.conf placed there by the step. Clues for dealing with quik are available at http://penguinppc.org/projects/quik/.
To boot back into MacOS without resetting the nvram, type bye at the OpenFirmware prompt (assuming MacOS has not been removed from the machine). To obtain an OpenFirmware prompt, hold down the command-option-o-f keys while cold booting the machine. If you need to reset the OpenFirmware nvram changes to the MacOS default in order to boot back to MacOS, hold down the command-option-p-r keys while cold booting the machine.
If you use BootX to boot into the installed system, just select your desired kernel in the Linux Kernels folder, un-choose the ramdisk option, and add a root device corresponding to your installation; e.g. /dev/hda8.
On G4 machines and iBooks, you can hold down the option key and get a graphical screen with a button for each bootable OS, Debian GNU/Linux will be a button with a small penguin icon.
If you kept MacOS and at some point it changes the OpenFirmware boot-device variable you should reset OpenFirmware to its default configuration. To do this hold down the command-option-p-r keys while cold booting the machine.
The labels defined in yaboot.conf will be displayed if you press the Tab key at the boot: prompt.
Resetting OpenFirmware on G3 or G4 hardware will cause it to boot Debian GNU/Linux by default (if you correctly partitioned and placed the Apple_Bootstrap partition first). If you have Debian GNU/Linux on a SCSI disk and MacOS on an IDE disk this may not work and you will have to enter OpenFirmware and set the boot-device variable, ybin normally does this automatically.
After you boot Debian GNU/Linux for the first time you can add any additional options you desire (such as dual boot options) to /etc/yaboot.conf and run ybin to update your boot partition with the changed configuration. Please read the yaboot HOWTO for more information.