Just bought a new iMac, I am in screen saver mode and have forgotten the password ?

This link will tell you exactly how to reset your password

http://support.apple.com/kb/HT1274

In OS X Lion or later, you can reset your user password with your Apple ID.

In Mac OS X v10.6.8 or earlier, follow these steps to reset a password when there is only one administrator account on the computer, or if the original administrator account needs a password reset. "Original" administrator account refers to the one that was created immediately after installing Mac OS X. If the original administrator password is known, that administrator account may be used to reset the passwords of other administrator accounts using the steps described above. These steps require an optical drive; if your Mac does not have an optical drive and you have a Mac OS X v10.6 Install disc, see below.

1. Start up from a Mac OS X Install disc (one whose version is closest to the version of Mac OS X installed). Usually, you can start from the disc by putting it in your computer, restarting, and holding the C key. Or, put it in the computer and click the Install or Restore icon you see in the disc's main window (after which the computer will start from the disc without you needing to hold C). Or, you can use Startup Manager or the Startup Disk preference pane to select the Install disc.
2. Choose a language, click the arrow button to continue,
3. Mac OS X v10.6 or later: Choose Password Reset from the Utilities menu.
   Mac OS X v10.5 or v10.4: Choose Reset Password from the Utilities menu.
   Mac OS X v10.3: Choose Reset Password from the Installer menu.
   Tip: If you don't see this menu or menu choice, you're probably not started from the disc yet.
   
   Note: The default keyboard layout is U.S. English while started from the installation disc. If you use a keyboard layout other than U.S English, use the Input menu (the flag icon on the right side of the menu bar) to select the desired layout before typing a new password.
4. Select your Mac OS X hard disk volume.
5. Select the user name of your original administrator account.
   Important: Do not select "System Administrator (root)". This is actually the root user. You should not confuse it with a normal administrator account.
   
6. Enter a new password.
7. Click Save.
8. Click the red button in the upper left corner to quit the application.
9. Choose Quit Mac OS X Installer... from the Mac OS X Installer menu.
10. Click Restart.

After following these steps, you'll want to also follow the steps in this article. Mac OS X v10.6: Resetting the original administrator account password on a Mac that has no optical driveThese steps require a second computer that has an optical drive.Note: If the second computer is a Mac, make sure Mac OS X v10.5.3 or later is installed. If the second computer is a PC, make sure the DVD or CD Sharing Update 1.0 for Windows is installed.

1. Open "Remote Install Mac OS X" from the Utilities folder.
2. Follow the onscreen instructions.

For more information, see Sharing DVDs or CDs with Remote Disc.