imac late 2009

Insert a compatible Mac OS X install DVD, restart with the C or Option key held down, use the Disk Utility to erase the internal drive, and install a fresh OS.

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How to erase a disk for Mac - Apple Support

1. Start up from macOS Recovery.
2. Open Disk Utility from the macOS Utilities window.
3. From the sidebar in Disk Utility, select Macintosh HD (or whatever you've named your hard drive).
4. Click the Erase button or tab, then complete these fields: Name: Enter a name for the disk or volume, such as "Macintosh HD". Format: Choose either APFS or Mac OS Extended (Journaled) to format as a Mac volume. Disk Utility shows a compatible format by default. Scheme (if shown): Choose GUID Partition Map.
5. Click Erase to begin erasing.
6. Quit Disk Utility when done. You can now install macOS.

No problem, easy if you've done it a dozen or more times. 🤔😕😧😵

For starters a Late 2009 21.5" iMac originally came with a 10.6.1 Snow Leopard 2 disc set.

https://everymac.com/systems/apple/imac/specs/imac-core-2-duo-3.06-21-inch-aluminum-late-2009-specs.html

In order to set the iMac back to the "original factor settings" you would need that original 2 disc set. In addition you also need a working SuperDrive or an external USB DVD drive.

Good news though' you can use a Retail Snow Leopard Upgrade DVD to Erase and Reinstall 10.6. Snow Leopard.

Mac OS X 10.6 Snow Leopard - Apple

Update' the DVD version to 10.6.8 using the combo updater.

Mac OS X 10.6.8 Update Combo v1.1

Then finally' go to the App Store and upgrade to the latest macOS your iMac can run.

How to upgrade to OS X El Capitan - Apple Support