It really helps to know the exact model of the Mac since some Macs may have slightly different options & procedures available to them. You can get the exact model by entering the system serial number here:
Check Your Service and Support Coverage - Apple Support
For an Intel Mac you will want to use Disk Utility to erase the whole physical drive as GUID partition and APFS (top option) if you are reinstalling macOS 10.14+. Otherwise you want to erase the whole physical drive as GUID partition and MacOS Extended (Journaled). You may need to click "View" within Disk Utility and select "Show All Devices" before the physical drive appears on the left pane of Disk Utility. The physical drive is usually shown by its make & model.....something like "Apple SSD A0256....". Don't worry about the "partition" tab, since it is not what you need here.
If you are booting to a macOS 10.10 or earlier installer, then you would need to use Disk Utility to partition & format the drive, but it doesn't sound like this is the case here. So if you are installing macOS 10.11+, you need to use the "Erase" tab within Disk Utility instead of the "Partition" tab.
Here is an Apple article with instructions for reinstalling macOS:
How to reinstall macOS - Apple Support
If this is a 2018+ Mac, then a bootable USB installer is not an option if the internal SSD has already been wiped since there is no way now to authenticate with the T2 security chip in order to modify the security settings to allow booting from USB (even if it was previously enabled, I don't think it is possible to boot to USB after the internal SSD has been erased, but you could try).