Q: macbook Pro 2012 set up

I just got my Macbook Pro 13.3 2012 in today from a previous owner. The machine has El Capitan in it.

How do I set it up for my apple account? No Setup assistant. When you turn it on the User Name is: Mac Owner and the password is the enter button.

How do I set it up for my apple account-change the User Name and password? How do I go about and do that?

I got Square Trade warranty with it, but I will be registering it with Apple for Apple care.

I need some help with this.

Carol