Given an older MacBook. Installed Sierra but it also restored the old owner's account

So it's asking for her password which I'm positive she won't give me. Completely understandable.

So how do I wipe everything again and start from scratch? At first when I was trying to install an OS, I kept getting the error message "no package available to install" or something like that, so I looked at threads here and came across the suggestion to change the date via Terminal, which I did, and it worked. So now my new problem is being able to use it and switching it over to my account.