Wipe all data from a late 2009 iMac with High Sierra

Make sure to follow @BDAqua's advice for disassociating your AppleID from the iMac as well as confirming you have all your important files on the new computer or external media.

If your iMac only has an SSD, then the simple erase @BDAqua mentions will be enough to destroy all data on the SSD due to how SSDs work.

If the iMac uses a hard drive or Fusion Drive, then you definitely want to secure erase the hard drive by writing a single pass of zeroes to the entire hard drive (not necessary for an SSD). While you can do this using Disk Utility while booted into Recovery Mode or from a bootable macOS USB installer, another option would be to create & use a bootable ShredOS USB stick. If the iMac has a Fusion Drive, then don't forget to use Disk Utility to perform a simple erase of the SSD.

Another option would be to first enable Filevault while you can still boot the iMac normally. Let the Filevault encryption process finish. Then when you boot into Recovery Mode or from a bootable macOS USB installer, you can just use Disk Utility to perform a simple erase to destroy the Filevaulted volume which destroys the enryption key to unlock the encrypted Filevaulted volume.

First...

If your Mac doesn't have Erase All Content and Settings

If the Erase All Content and Settings feature isn't available on your Mac, follow these steps instead. Before you begin, it helps to know which macOS your Mac is using.

1. Back up or transfer your files

If you don't have a current backup, you can back up your files to an external storage device or transfer your files to a new Mac.

2. Sign out of iTunes

If using macOS Catalina or later, you can skip this step.

1. From the menu bar in iTunes, choose Account > Authorizations > Deauthorize This Computer.
2. Enter your Apple ID and password and click Deauthorize. Learn more about deauthorizing.

3. Sign out of iCloud

When you follow these steps to sign out of iCloud, you're asked whether to keep a copy of your iCloud data on this Mac. It's faster to just keep a copy, because you will erase your Mac later. 

macOS Ventura or later:

1. Choose Apple menu  > System Settings.
2. Click your name (Apple ID) in the sidebar.
3. On the right, scroll down and click Sign Out.

macOS Monterey, macOS Big Sur, or macOS Catalina:

1. Choose Apple menu  > System Preferences.
2. Click Apple ID.
3. Select Overview in the sidebar.
4. Click Sign Out.

Earlier versions of macOS:

1. Choose Apple menu  > System Preferences.
2. Click iCloud.
3. Click Sign Out.

4. Sign out of iMessage

1. Open the Messages app.
2. From the menu bar in Messages, choose Messages > Settings (or Preferences).
3. Click iMessage.
4. Click Sign Out.

5. Unpair Bluetooth devices (optional)

If your Mac is paired with a Bluetooth keyboard, mouse, or other device that you plan to keep, you can unpair it. This optional step prevents accidental input when the Mac and device have separate owners but remain in Bluetooth range.

 If you do this on a Mac that doesn't have a built-in keyboard or trackpad, you will need to plug in a USB keyboard and mouse to complete the remaining steps. 

1. Choose Apple menu  > System Settings (or System Preferences).
2. Click Bluetooth.
3. Move your pointer over the device that you want to unpair, then click the Disconnect button or remove button (x) next to the device name.

Finally...

Open System Preferences>Accounts, unlock the lock, click on the little plus icon, make a new admin account, log out & into the new account.

In the same pref pane highlight your old account, click the little minus icon, then use Disk Utility to Secure Erase Free Space.