If you are running an AppleMacintosh you might be running OpenFirmware with a password and don't know the password. Normally you press C to boot off CD-ROM (needed to install Linux) but this might be locked out. If so you can remove the password with this program provided you have MacOSX root access.
If you are working on NewWorld? firmware then you need a boot manager such as yaboot. You will need to create an Apple_Bootstrap type partition (8 MB should be fine). Your distribution should automatically take care of much of this but IanMcDonald found only Debian unstable got it right on an emac.
If you want to build your own kernel and share code tree with i386 have a look at the O option when building. (Do make help for more documentation). The image will be called zImage rather than bzImage by default and remember to build support for the disk controller and file systems as built in rather than modules to enable you to boot (or alternatively use initrd(4)). To update yaboot on Debian or other similar distros edit /etc/yaboot.conf and then type ybin.
In /etc/yaboot.conf make sure that the label has no spaces in it or else it won't display in the boot manager