You generally want to give an interviewer as much information about you as possible. However there are some questions that you should not, or do not have to, answer under US law.
Work status and citizenship.
Interviewers cannot ask if are you a U.S. citizen. They cannot ask where are you from or where were your parents born. They also cannot ask what is your native language is.
They can however ask if you are you authorized to work in the U.S. and what languages do you speak if it is relevant to the position.
Marital status.
Interviewers cannot ask if you are married or divorced. They cannot ask where your spouse works. They cannot ask if you have children or are you planning to have children.
Age.
Interviewers cannot ask how old you are or when were you born. They also cannot ask how long have you been working.
Interviewers cannot ask how old you are or when were you born. They also cannot ask how long have you been working.
If it is relevant to the job (for example working in a bar) they can ask if you are you at least 18 or 21 years old.
Disability.
Interviewers cannot ask if you have any disabilities or medical conditions. They cannot ask how your health is or if you take any prescription drugs. They also cannot ask about mental illness, alcoholism or if you ever been in rehab.
They can ask if re you able to perform this job with reasonable accommodation or if you have any conditions that would keep you from performing this job.
Note: Employers are allowed to require that a candidate pass a medical exam relevant to the responsibilities of the job and to pass a drug test.
Religion.
Interviewers cannot ask what your religion is.
Interviewers cannot ask what your religion is.
They can ask whether you can work on weekends if the position requires it.
What to do if you are asked one of these questions.
If you are asked one of these questions you have a few choices. First, you can just answer it if you think the interviewer is simply trying to get to know you. If you are comfortable answering the question then just go ahead and do it. Secondly, you can avoid the direct question and reassure them that your personal life won't interfere with your work. Lastly, if you feel comfortable doing so, you can question the relevance of the question. That might get the interviewer to ask for the information in a more appropriate way.
A career coach can help you prepare for an interview.
You might also be interested in reading: How to take your age out of the equation when applying for a job.
