How do I reset the password for my 2009 iMac?

Hi there

Please follow this instructions

Steps to Reset the Password

• Shut down your Mac: Turn off your iMac completely.
• Boot into Recovery Mode: Turn on the iMac and immediately press and hold Command + R until you see the Apple logo or a loading bar.
• Open Terminal: Go to the top menu bar, click Utilities (or Tools), and select Terminal.
• Run the reset command: Type resetpassword into the Terminal window and press Return
• Follow the prompts: A Reset Password window will open. Choose the option for forgetting your password or follow the onscreen instructions to enter a new password.
• Restart: Once finished, restart your iMac from the Apple menu and log in using your new password. 

If you have the original Install Disc set that came with that iMac..?

You can use Utilities on Disc 1 to reset the iMac's password.

If not, then you need to use whatever steps work in the following support article.

If you forgot your Mac login password - Apple Support