You may have inherited a Mac from an older sibling, or purchased one from eBay or Gumtree. It's simple enough to reset to factory and sign back in as yourself using your own Apple ID with your own settings.
First you'll need to turn off your Mac. Then press the power button and immediately hold down the Control and R keys until you see the Apple logo or spinning globe icon.
Release the keys and shortly afterwards you should see the macOS Utilities window appear.
In this are four options: Restore from Time Machine Backup, Reinstall macOS, Get Help Online and Disk Utility. Click on the last one and then select Continue.
Next, you'll see a list of all the HDs currently attached to your Mac. In the left columns are the names, while in the main panel are the details of the selected drive.
Look under the Internal section for your main drive. This is often called Macintosh HD and appears at the top of the list. Click on it so that it's highlighted, then click the Erase button from the options in the main panel.
A popup window appears with a few fields you'll need to complete. The first is Name, in which you should enter the title you wish to use for your new drive.
Under this are two dropdown menus. The first is Format, from which you should select Mac OS Extended (Journaled), while the second field is called Scheme and in here you need to select GUID Partition Map.
Finally, click Erase and the drive will be purged of all users, apps and content.
Of course, to be able to use your Mac you'll need to reinstall the operating system. To do this go back to the macOS Utilities menu, select Reinstall macOS and follow the onscreen instructions.