How do I do to install macOS High Sierra in my Mac Pro 2009 Desktop from scratch? Thank you in advance!

Internet Recovery is NOT available in your model’s ROM, so you will need to explore this list of other possibilities.

When your computer was released, the way you launched the required Utilities (including Disk Utility and Installer) was to use the ones on the Release software DVD. if you have a model-specific version for your model (unlikely) or a Full Retail 10.6 DVD, you use its Utilities, boot and install that version, then use Software update to get to 10.6.8 with all updates, which is the version that can reach out to the Mac App store and download and install a later version.

10.11 El Capitan is a recommended waypoint, even if you expect to install a later version, because it has an improved Mac App Store that makes getting later versions much easier.

The next source of Utilities is the Recovery Partition on the boot drive. If your drive spins up, even if not MacOS bootable, it may still have a usable recovery partition. To get there, try invoking recovery with Command-R or hold Alt/Option at startup and see if the recovery partition shows as a potentially bootable drive.

Recovery Partitions up through 10.12 Sierra can be found with the Startup Manager (Alt/Option boot). At 10.13, if an SSD boot drive is used, the format is transitioned to APFS. The Recovery partition is present, but it is inside the APFS volume structures, and the Startup Manager on an older Mac can not find it.

The next source of Utilities to consider is any MacOS 10.6 or later versions on any additional drives or clones you may (or may not) have lying about, even if they are from another Mac. You can use those Utilities to ERASE a new drive, and start the installer to place MacOS on the new drive.

The next source to consider is a Time Machine backup drive. Versions from 10.7.3 or later are said to contain a Recovery Partition that could be used to ERASE a new drive and run Installer to place MacOS on a new drive.

Two Mac solutions:

With certain combinations of new and old Mac, you can use Target Disk mode to repair, erase, and install on the drive of the old Mac, by treating it as a disk drive on the new Mac.

Transfer files between two Mac computers using target disk mode - Apple Support

IF you have a different Mac, you can use it to download MacOS install image, then interrupt the process and create a BOOTABLE USB-stick Installer/Utilities stick. BOOTABLE is key, because the way you will install from this USB-Stick is to BOOT the USB-stick, and use its Utilities to ERASE your drive and start the Installer. here is the article on bootable USB-Stick Utilities/Installer:

What you need to create a bootable installer

• A USB flash drive or other secondary volume formatted as GUID partition Map, Mac OS Extended, with at least 14GB of available storage

• A downloaded installer for macOS Big Sur, Catalina, Mojave, High Sierra, or El Capitan.

from:

How to create a bootable installer for macOS - Apple Support

Create a bootable installer for macOS - Apple Support

NB>> if you name your incoming USB stick exactly MyVolume, you can copy and paste the very long Terminal command from the article directly into the Terminal window, without having to change anything.

You will need to use another Mac to download high Sierra from the page that you came here from, and make a bootable usb with high sierra

1. Plug in the USB flash drive or other volume that you're using for the bootable installer. 
2. Open Terminal, which is in the Utilities folder of your Applications folder.
3. Type or paste one of the commands below into Terminal, then press Return to enter the command. Each command assumes that the installer is in your Applications folder, and MyVolume is the name of the USB flash drive or other volume you're using. If the volume has a different name, replace MyVolume in the command with the name of your volume.
4. When prompted, type your administrator password. Terminal doesn't show any characters as you type. Then press Return.
5. When prompted, type Y to confirm that you want to erase the volume, then press Return. Terminal shows the progress as the volume is erased.
6. After the volume is erased, you may see an alert that Terminal would like to access files on a removable volume. Click OK to allow the copy to proceed. 
7. When Terminal says that it's done, the volume will have the same name as the installer you downloaded, such as Install macOS Monterey. You can now quit Terminal and eject the volume.

See this link for the commands you need

Create a bootable installer for macOS - Apple Support

The most pointed answer depends on exactly WHY you want to re-install, AND what version you are installing OVER, i.e., what MacOS is running now.

By design, macOS install does not disturb any of your files, and only bulk-erases locked system directories. So a re-install in place is completely possible. However, if your drive stating to have troubles, re-writing the required over 350,000 files can push a marginal drive into outright failure.

MacOS will never install an OLDER macOS over a newer MacOS.

In each and every case, Best Practice is to have a Trusted Backup at hand, to restore your files after installation, if needed.

¿what is the date of your most recent backup, and by what method?