Having to have an interview to get a visitor visa is standard - it is nothing to worry about. This is just one of the ways that the embassy/consulate will check you out to assess whether to give you a visa. Its main purpose is to make sure that you do just want to visit the United States on a temporary purpose so the interview may be used to check your ties to your home country (i.e. do you own property, are you employed, where do your family members live and so on).
It is still worth putting some effort into your interview - if you don’t get it right then it could well have an impact on whether a visa is issued or not. So, make sure to turn up on time, be polite and to look good physically - you don’t need to dress formally but make sure you are have a wash and brush up before you go!
Before your interview you will be told what documents to fill in/take with you. Make sure that you have everything you need and read it through before the interview so you can remember what you wrote on forms/documentation. If you find that the interview will be held in English and you are not 100% comfortable with the language then you can ask for an interpreter to help you out.
The actual interview process will vary from country to country and may change according to your circumstances (i.e. whether you are applying for a visa yourself or are being sponsored). Part of the interview may well cover checking your application and supporting documents with you.
It is likely in most countries that you will be asked some other questions. Again, these questions are designed to check that you are intending to visit the U.S. on a temporary basis and that you will return home thereafter. The number and type of questions you will be asked will vary from country to country and on your type of application.
The following list of questions will give you a guide as to the kinds of thing to expect here:
These questions may seem a little personal - just remember, that they are just designed to try and help the official check that you do intend to return home after your trip and that you aren’t using a visitor visa to try and relocate to the U.S. full-time.
Bear in mind that security is high nowadays and you may be asked to go through security checks before your interview and you will be finger-printed in some countries.
In some cases the official will be able to tell you on the spot whether your application has been approved or denied. In others (such as cases where checks need to be made in the U.S.) you may have to wait a few weeks to find out the decision and may have to return for re-interview if an issue arises.