how can i get past the authentication code stuff if i bought from a used computer vendor [closed]

Unless you need data stored on this used machine, or the machine comes with a custom configuration for special tasks, it is strongly advisable to remove the preinstalled OS for the following security reasons:

• User name and password of the administrator account are different to your needs.
• Administrator credentials are known to other people.
• Individual configuration of the machine is unknown.
• There is no guarantee that the machine comes without malicious software.
• Partitioning may be different to your needs.
• The OS may not be up to date.

Reinstalling a fresh and untouched ("vanilla") Ubuntu is very easy and should not take you more than 30 Minutes:

• How do I install Ubuntu?