Using a pre-owned Mac
You can simply create a new Admin account in the system. After creating it, during the initial setup, you'll be asked to sign in with your Apple ID. You can also choose to create a new Apple ID if you don't have it already.

Apple ID is required to interact with the plethora of online services offered by Apple. You can learn more about Apple ID in the Apple Support document:


Create, manage, and use an Apple ID


Once you have your account setup, you can use the system normally and will have access to all the features. Your Dad's user account can stay on system safely.

To create a new admin account, go to System Preferences → Users &amp; Groups → (Click the lock to make changes) → (Click on + button towards the bottom of the left sidebar) → New Account → Administrator.



Creating an additional user account won't take up much space. It gives you a segregated environment while giving access to the systemwide pre-installed apps (those installed in /Applications directory). You generally won't face any issues later while repairing or installing things.