First things first. Be confident.
Lab coat is not necessary as you are only going to appear for an interview. Just dress yourself in formals with a tie. With winter around the corner, you can also use a blazer (navy blue will be better).
As far as preparation for interview questions in concerned, I presume you are already good with academics as you have cleared the written test. Apart from the academics, prepare yourself good with general questions like, introducing yourself, what do you know about the institute, why did you choose this job, where do you see yourself in some years from now etc.
Also, if you have mentioned any hobby in you resume, expect some questions from there as well.
In case you have some work experience, or have done internship or have been involved in any project work, prepare more on this front.
Here if you have not mentioned if the institute, for which you are going to appear for interview, is government or a private institution. In case it is a government institute, you can also expect some general knowledge questions.
Do take a look on interview day newspaper to take a not on any news belonging to your industry.
Try not to look tense or nervous (as you might be as this is your first interview) and wear a gentle smile. Even if you come across some questions, do not get nervous as it will affect your response on next questions.
At last, an interview day is just another day, so don't feel down if you do not fare well in interviews.