If my iMac A1311 hard drive is totally erased, how can I reinstall is x software

If you truly wiped the entire drive clean see if you can find your Snow Leopard installer discs, or buy a new set. That's a 2009 iMac so you shouldn't have the problems with a retail set that a newer model that originally came with Snow Leopard would encounter.

If you tend to work with older Macs then get into the habit of having bootable clone copies of your drives so you can use them to install old systems.

The answer largely depends on what OS version it was previously running, and what happened to the drive.

Recent OS versions have maintained a small 'Recovery' partition on the drive. This can be used to boot to a minimal system and reinstall the OS should something catastrophic happen to the main system volume (such as it getting erased).

Hold Command-R during power on to attempt to boot to the recovery partition. Then follow the on screen instructions to reinstall the OS.

However, if the entire drive was erased, not just the system or data partition, then the recovery volume may have been erased too, in which case this won't help. In this case the easiest path is to download an OS installer to a USB drive and boot your system from this. Instructions can be found at: https://support.apple.com/en-us/HT201372