If none of the content of the Mac is of any use to you, then your best option is to do a “complete reset.” Make sure you are connected to wifi, then boot into Recovery (holding down command-R immediately on power-up).
When Recovery comes up, start Disk Utility and erase/reformat the drive. From your description of the Mac, choose HFS+( (Mac OS Extended (Journaled)). Another unknown here is whether your drive is a hard disk or SSD. But if you wish to upgrade to Catalina or newer and your startup drive is SSD, you should choose APFS. You did not specify your Mac model so I cannot tell if it is upgradeable to at least Catalina.
By the way, if formatting your drive as APFS, you shouldn’t use older versions of Disk Utility. The earlier versions, starting from Mojave, were the early generation of APFS, which were very bad indeed. It keeps on getting better with the latter versions of macOS. But even with the latest macOS, if possible, use Disk Utility from Recovery since it would be pretty much the newest version from Apple, loaded over the internet.
After the formatting, quit Disk Utility and from the main window, select to install (whatever macOS version is offered to you). This indicates the newest macOS version your Mac can run. You can then proceed with the installation, setup, etc.
If you did format the drive as HFS+ and the offered upgrade is macOS Catalina or newer, you should go back to Disk Utility and reformat the drive as APFS.
