How do I erase my hard drive on an older iMac running 10.3.7

Startup from the original install disc that came with that iMac.

It goes something like this, insert the install disc, restart holding the C or Option key to startup from the disc, navigate to Utilities on the menu bar, open Disk Utility, select the Macintosh HD and click the Erase button.

I would add that you should click on the "Option" button within Disk Utility and select a secure erase option to write a single pass of zeroes to the entire hard drive so that your personal data is overwritten if you are planning on selling or recycling the iMac. If you are reinstalling macOS for yourself, then just ignore my post.

Good work so far, you’re about 90% done.

Go back over to the side bar and select the Macintosh HD, click the Erase tab along the top, name it Macintosh HD, set the Partition Map Scheme to GUID, the Format Mac Extended (Journald), then hit the Erase button in the bottom right corner.

You may instead need to use the Partition tab and using the instructions in this article instead to prepare the drive for macOS:

https://eshop.macsales.com/assets/support/support-formatting-and-migration/Mac_Formatting_6-10.pdf