What're the things I should never do during the period I'm employed at company?

Don’t talk about politics at work. Don’t put the employer above yourself. Don’t gossip and criticize other employees, and try to see the good in people. Don’t stop looking for other better jobs. Don’t neglect maxing the company match for your retirement.

Leaking the security certificates and administrative credentials to an external party. Using the earlier customer payments to pay future customers.