You could start it in Firewire target mode, which means it will start up as if it was a simple Firewire disk without a processor, etc.
To do so, you must connect it to your MacBook via Firewire and then start it up while holding the "T" key.
It should then show up on your MacBook as a Firewire disk, and you will be able to copy anything from there to your eMac.
System software could then be installed using (e.g.) a disk image.
If you have a physical system disk (e.g. burned from a system disk image), you can also start your eMac by inserting this disk and starting it up while holding the "C"-key. It will then start up from your system CD or DVD. After system installation from this CD, restart your eMac and you're done.
System: MacOS 10.4 "Tiger" is best (Mac software like "Mail", "Safari", etc. is included in Tiger system disk)
other software: any PPC G4 compatible software
Use "Software Update" in the Apple menu to get the most recent versions of Mac software.