default user account for mac book

Go to the Accounts system preference, click on the lock icon and enter in your user ID and password so you can make changes, click Login Options, and in the "Automatic Login" pulldown change it to your account or to Off, whichever you prefer.

I would suggest you remove that former user's account, unless there's some reason you need to keep it. You never know what sort of cruft or problems might be lurking. I actually usually advise anyone purchasing a used computer to erase the drive completely and reinstall the OS, just to make sure you're not inheriting someone else's problems or, in an extreme case, some malware such as a keylogger.

Regards.