Most states operate under the "at-will" employment doctrine, which provides that an employee works for an employer at the will of the employer.
This essentially means that an employee can be fired at any time for any reason except for those expressly prohibited by law, such as discrimination (note that very small employers may be exempt from this) or in retaliation for whistleblowing or for exercising certain legal rights such as taking family and medical leave, military leave, or time off to vote or serve on a jury.
If you don't have something orally or in writing from your employer that states otherwise, you will most likely be an at-will employee.
On the other hand, if your employer has provided you a policy that requires good cause to terminate the employment relationship, pay special attention to the reasons listed and, of course, avoid violating the policy.
With the thought that "an ounce of prevention is worth a pound cure," here are five things you should always do to help ensure that you remain gainfully employed.
1. Do show up
Absenteeism is probably the most easily justified reason an employer can fire you, particularly since evidence supporting the claim, e.g., timesheets or time cards, isn't difficult for employers to compile. If you have medical reasons for missing work, be sure to keep documentation as your employer may be required to attempt to accommodate the situation.
2. Do work while at work
Employers want competent, productive employees, and there's no better way to show how great you are at your job than to actually work.
This means cutting down on coffee chat breaks as well as email and instant messaging, particularly if you're communicating with coworkers, as certain forwarded jokes may not strike everyone in the same way. If your well-intended Internet humor goes awry and a coworker is offended, not only may you be reprimanded or even fired, your employer may be in for a lawsuit as well.
3. Do play well with others
Respect is essential in the workplace.
As in life, it's best to follow the tried and true Golden Rule of treating others as you would like to be treated.
Physical or emotional abuse towards your coworkers or management can be grounds for immediate termination, as can disrespect and refusal to carry out orders, which is often termed "insubordination."
4. Do play by the rules
Although many employers don't have specific policies regarding your behavior outside of the workplace, if yours does, be sure to read them and understand what is expected of you.
If your employer has informed you of clear policies that can withstand legal challenges, any violation of those rules can be grounds for immediate dismissal. Note that criminal activity such as embezzlement or fraud and coming to work intoxicated or high are often good cause for termination.
5. Do keep copies of contracts, policies, time cards, etc.
If your workplace provides contracts, policies, time cards, and other written documents regarding your employment, keep copies of these somewhere safe so that you can refer to them if needed. Additionally, if you have been given information orally, specifically about potential reasons for termination, write them down and date them so that you can remember exactly what was said.