It's perfectly fine to be nervous. Try to think of ways to break the ice. Even 30 seconds of small talk while entering the room will do a lot to comfort you. You might even say something along the lines of "Sorry if I seem a bit nervous, I'm just really excited about the job" (see point #2 below)
A job interview is really only about :
- Is this person qualified for the job
- Will this person like the job (to be engaged and stay s long time)
- Will this person fit well in the team
#1 and #2 require preparation. Read the job description thoroughly, browse around at the company's web page, Google recent news articles, etc. and formulate good answers to common questions and write down examples that prove your point (see some of the other posts).
#3 can be a bit tricky to research, but can be done. Look at social media posts to get a feel for the culture to understand dresscode (both over dressing and under dressing can be problematic), understand industry jargon (and avoid controversial jargon/statements), etc. Understand this, but be true to yourself. Don't wear flip flops for McKinsey interview and don't wear a tuxedo for a handyman job.
*temporarily liberated