[Sterz]

I got an old eMac from my school, but it has no software therefore it won't even start past a certain screen. How can I put software on it and what software should I use?
Is there a way I can put it on by connecting a MacBook Pro?

[Dex13]

depends on what emac you have, it will either be 10.4 or 10.5 that you will be able to install.
(How to Identify your Emac)

all you need to do is get a install disc for the appropriate os and start up holding down "C" on the keyboard.

you should probably wipe the hard drive while you're at it.

[Whizzbizz]

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.

Software:
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.

[P]

You need a version of the operating system that is compatible with that computer. Depending on the exact model, the newest you can use is either 10.4 or 10.5. Search eBay to see if you can find a used copy.

[Waragainstsleep]

You can't install from a disk image on the MacBook Pro as whichever machine is doing the installation needs to boot from the installation disk and the MBP will never boot a PPC OS.

Schools usually keep the software discs and if they had several eMacs they should have a pile of discs that will work. Worth asking around. Someone is bound to know where they might be. They will be grey discs with white text in white paper sleeves with a cellophane window in them.