1. It may or may not.
2. You can purchase 10.5 "Leopard" from eBay. Make sure the DVD you purchase looks like this:
Leopard would be the best OS for your G4 as it is the latest it can run, however if you want a bit more of a slimline experience Tiger is the one to go for. This is cheaper and can also be found from eBay.
Make sure you NEVER buy plain grey disks as these are machine specific. Disks with an X on are OK.
Make sure you have the maximum RAM which is cheap these days. Go to these links to determine an approximate price and for instructions.
2. Hold apple key + s key down after you hear the chime. (command + s on newer Macs)
3. When you get text prompt enter in these terminal commands to create a brand new admin account (hitting return after each line):
mount -uw /
shutdown -h now
4. After rebooting you should have a brand new admin account. When you login as the new admin you can simply delete the old one and your good to go again!