Hi I am in the same situation as most of the people above. I bought an iMac 2nd hand and the previous owner had installed Windows 10 as the operating system. I wanted to reset it and install OS X so I thought it was a good idea to format the drive so I can do a clean install. If you are resetting your Macbook to factory settings, all applications (except for the ones that came with the Mac) will be erased, including your MS Office (Word). However, if you have the original disk, or product key card for online download, or it is linked to your Microsoft account, you can reinstall it once you factory reset your Macbook. Erasing your Mac permanently deletes its files. If you want to restore your Mac to factory settings, such as to prepare it for a new owner, first learn what to do before you sell, give away, or trade in your Mac. Then erase your Mac as the final step. Use Disk Utility to erase your Mac. Plug the bootable installer into a Mac that is connected to the internet and compatible with the version of macOS you're installing. Press and hold the Option (Alt) ⌥ key immediately after turning on or restarting your Mac. Release the Option key when you see a dark screen showing your bootable volumes.
These steps apply only to Mac computers with an Intel processor. The steps to erase a Mac with Apple silicon are different.
Your built-in startup disk should be the first item listed in the Disk Utility sidebar. It's named Macintosh HD, unless you changed its name. If you don't see it there, choose Apple menu > Shut Down, then unplug all nonessential devices from your Mac and try again.
If your disk still doesn't appear in Disk Utility, or Disk Utility reports that the erase process failed, your Mac might need service. If you need help, please contact Apple Support.
For more information about using Disk Utility, see the Disk Utility User Guide.