I forget my screensaver user password on my iMac

Use the Recovery Partition — Starting with 10.7 Lion, which was sold only through the Mac App Store, the installer disc was replaced by the Recovery partition, a small chunk of the boot disk that contains a stripped-down version of Mac OS X and essential utilities. To reset the administrator password when running Lion or later:

1. Restart the Mac while holding down the Option key, and double-click the icon for the Recovery partition. A Mac OS X Utilities screen appears.
2. Choose Utilities > Terminal.
3. In Terminal, type resetpassword. Rather unusually for a task performed from the command line, a graphical Reset Password window appears.
4. Select the startup volume at the top of the window, and then choose a user account from the pop-up menu. In the fields below, enter the new password, confirm it, and add an appropriate hint.
5. Click Save, and then choose Restart from the Apple menu.