We all run into interviews occasionally where they ask questions we can't answer. If they are asking about your past work experience, and it's something you never did, you can try to think of something similar. For example, I did not have experience doing public speaking, such as teaching a class, but, I had presented a press conference (crazy, huh?). If they ask how you will handle a particular situation, and you honestly don't know, try to address the common steps you would take in reaching your decision. For example "I would do it in the way I had been trained, following the company's policies and procedures. I would make sure that whatever I did was carried out in a safe manner. If I was not sure of what to do, I would discuss it with senior co-workers."
If they ask you, "tell me about a time when. . . " types of questions (tell me about a time when you had a difficult customer), set up your response using the STAR format:
Situation: what was the situation?
Task: what was it you needed to accomplish?
Action: what did you do?
Results: what was the outcome?
The employer is not always looking for answers, believe it or not! Sometimes they want to see your problem-solving skills. An employee should not always run to the supervisor for everything, as the supervisor is trying to get stuff done! Co-workers, reference resources, etc, all come first, but, at the same time, if there is a customer waiting for your response, you don't want to spend too much time doing research! So common sense comes into play.
Now, if you find you are truly way in over your head at the interviews, there is a possibility that you are applying for the wrong types of jobs. If that is the case, spend more time reading job postings before you apply. Be more selective in the types of jobs you apply for!
Let me know if you have additional questions - always happy to help!