Ask your parent, sibling, or best friend to be the interviewer, and give her or him a list of questions to throw at you, especially the hard ones . There are ways to handle each of these. And remember, if you get a question that you can't answer, simply say you don't know. Then say the question is something to which you would like to give more thought and that you are willing to learn what it takes. Again, an employer will respect someone who is honest and open about his or her limitations.
Body language is the other thing to be well aware of.
Hand and arm movements shouldn't be too large. Your posture should be relaxed, but alert. Communicate interest and energy.