Q: Without a password how can I access the computer  and make it my own?

Follow the steps here:

What to do before selling or giving away your Mac - Apple Support

To restore it back to factory condition, and set it up as your own.

Correct. The Computer will be back to the way to was when it was first taken out of its box the day it was bought.

Not really true since the Apple article boots to the Recovery partition. When you boot to the Recovery partition and erase and reinstall OSX you install the OSX version that was previously on the Mac.

If you boot to Internet Recovery

If you can't start up from OS X Recovery, try holding down Option-Command-R to start up from OS X Internet Recovery*.

About OS X Recovery - Apple Support

and erase and reinstall OSX you get the OSX version that was on the Mac wan it was delivered.

lllaass wrote:

 

Not really true since the Apple article boots to the Recovery partition. When you boot to the Recovery partition and erase and reinstall OSX you install the OSX version that was previously on the Mac.

 

 

True,  but its as close as it can get. It may be a different version of the OS, but it will be wiped clean.