The easiest way to make an interview less nerve wracking is to just make a friendly but professional first impression. Doing that can help put both parties at ease and establish a tone that'll help make things go smoothly. Shake the interviewer's hand, smile, make eye contact, introduce yourself, speak clearly, and first and foremost be sure to pee on everything in the interviewer's office to quickly establish your dominance. Do that and I'm sure you'll be fine.