Mac Pro A1186 2138

Apologies, I read 2017 model, you clearly wrote 2007.

2007 Mac does Not contain any ROM-resident software that can assist you in any way. If you did not receive an Installer/Utilities DVD with it, doing ANYTHING with this computer will be difficult.

You need some source of an Installer. It originally shipped with 10.4.9 on a model-specific DVD. Model-specific DVDs (solid colors) for a different model will NOT work, because they do not have drivers for every appropriate model Mac. Full Color Full retail DVDs will work. You need to get to 10.6, and that can only be accomplished by using a 10.6 DVD. In this case, that means a "full retail" DVD.

Once you get to 10.6.8, and fully updated, the Mac App Store appears, and you can download El Capitan, then later versions if desired.

If you have another Mac, there are some other shortcuts that use your other Mac.

The "standard" way a Mac should be delivered to yours with its factory MacOS in place, and on first run, it should start up in Setup Assistant to allow you to enter an initial User and set it up.

If that has not been done, the computer is short of the standard, but if you want to proceed anyway, hold Command-R at startup and use your Wi-Fi or Ethernet connection to load the appropriate software to ERASE the new drive and proceed to Install. The initial chunk of code in the computers ROM allows it to reach out over the Internet and download the Utilities needed to Erase the drive and fetch a new MacOS image and start an installation. The procedure is not complex, but is not completely automatic:

How to reinstall macOS - Apple Support

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