Interviews can be stressful - no wait, they are stressful.  It's selling yourself to an employer, hoping that they'll take interest and hire you. 

Even more scary is that even if you pass the FIRST interview, there will be 2-3 more interviews after that.  There are various kinds of interviews that you could be facing.

Before an interview, do not arrive more than 20 minutes early as it can set the mood off.
Interview Types:

• Personal and Informal - One on one in an informal setting like a restaurant or just the work-site.  Usually a more much relaxed atmosphere. Business casual or casual clothing.
• Personal and Formal - One on one but in a monitored setting and a time frame.Professional attire is mandatory.
• Progressive Interview - Long interview process where you may be interviewed by more than one.  HR, Manager, Department Head, etc.  Each will be asking questions about yourself and your capabilities in performing your job, etc. Beware they may end up asking you repeated questions that have already been covered.
• Group Interview - Intimidating atmosphere, where you meet with a group of more than 3 people in a board-room setting.

Advice:

• Stay calm and focused - try to work off the nervous energy before arriving for your interview by taking a calming walk around outside, listen to soothing music, etc that can put you in a good mood
• Firm handshakes are great, and look them in the eye with confidence 
• During a personal and informal interview - try to engage a personable atmosphere with your interviewer
• Standardized questions like "What's your greatest weakness" will be asked.
• Try to form a connection with the person who interviews you by telling more about yourself that may appeal to them and make them want to hire you
• In a group setting, try to look at everyone in the eye and not just focus on one person - remember that you're being interviewed by a group and not just one person. 

Aside from that, here are 5 Ways to Turn off Employers.

• Arriving too early for an interivew
• Letting Desperation Show
• Being too Aggressive with Follow-Up
• Talking Crash with Anyone
• Lacking Direction