Never used a Mac before.

Hey Techspot,
Someone was giving away a free eMac, it's running OS X and the harddrive is loaded out, but when I try and run the install disc to reformat, it wants the password of the previous owner, which I do not know. Never used a Mac before, so, where do I go from here?

Try rebooting the machine and holding down "c" on the keyboard with the restore disc in the drive (or I believe you could hold down option, then choose to boot off cd). If that gets you to the installer menu, then you should be able to to a clean install from there. Also, if you can boot from the disc that way, you could reset the password on the machine, as shown here (http://docs.info.apple.com/article.html?artnum=106156) under "Resetting the original administrator account password."

1. Start up from a Mac OS X Install CD (one whose version is closest the the version of Mac OS X installed). You should first disable Open Firmware password protection, if it is enabled. Hold the C key as the computer starts.
2. Choose Reset Password from the Installer menu (or Utilities menu in Mac OS X 10.4 Tiger). Tip: If you don't see this menu or menu choice, you're probably not started from the CD yet.
3. Select your Mac OS X hard disk volume.
4. Set the user name of your original administrator account.
Important: Do not select "System Administrator (root)". This is actually a reference to the root user. Do not confuse it with a normal administrator account.
5. Enter a new password.
6. Click Save.

That did the trick, thanks.
Only problem now is I don't know what to do with .dmg files.

You double click them, and then they 'mount' as a drive. dmg is really just a disk image. Since dmgs usually contain applications, once the dmg is mounted you just drag the program to the Applications directory and then its installed, very simple and elegant. To uninstall you just drag the app to the trash.