restarting after hard drive replacement

You're fighting the which came first problem: The Chicken or the Egg.

You'll need to create a bootable external drive. If your original drive is still working you could install it into a FireWire HD case and then use it to boot up your system. You could also locate your recovery CD/DVD disk that came with your system.

If you can't get these to work for you you'll need to find a friend that has a Mac then prep up a USB thumb drive as a bootable drive with a copy of the OS installer. Then you can boot up your system and install a fresh copy of OS onto your new drive.