If you’re nervous, visit the restroom before your interview, and wash and dry your hands so they aren’t sweaty. Take some deep breaths.
During the interview, try to remain as calm as possible. Ask for clarification if you’re not sure what’s been asked and remember that it is perfectly acceptable to take a moment or two to frame your responses so you can be sure to fully answer the question.
Also, remember that thorough preparation helps build confidence and relieve stress. The more you research the company, practice answering interview questions, and prepare for the day of the interview, the calmer and more confident you will feel.