Short Answer 
To reset your Mac, first, restart it while holding the Command and R keys to enter recovery mode. Next, use Disk Utility to erase the startup disk, then reinstall macOS following the instructions provided, keeping in mind that this will erase all data.
Step 1: Restart Your Mac
Begin the process by navigating to the Apple menu and selecting Restart. As your Mac is restarting, immediately hold down the Command and R keys together. This action will help you enter the recovery mode when the Apple logo appears on the screen.
Step 2: Use Disk Utility to Erase the Startup Disk
Once in recovery mode, you will see the macOS Utilities window. Select Disk Utility and click Continue. In Disk Utility, find your startup disk, usually named “Macintosh HD”. Click on it, and then select the Erase tab where you will:
- Enter a new name for the disk.
- Choose Mac OS Extended (Journaled) as the format.
- Click Erase to initiate the process.
Step 3: Reinstall macOS
After successfully erasing the startup disk, return to the macOS Utilities window. This time, select Reinstall macOS and follow the on-screen instructions to complete the reinstallation process. Remember, performing this reset will delete all data on your Mac, so ensure you have backed up your important files before you begin.