Never go to an interview without researching the company. It is almost certain that one of the first questions you will be asked is "do you know what we do here?" And if you just shrug and tell them you have no idea you'll be on your way to the exit door!
At the very minimum look at their website, find out what their business is and who they work with. If you are going to the interview via an employment agency you should have been given a full brief on the job and the company, and if not, ask for one. If you applied directly to the employer you should have been told what the job entails, so do your homework and find out more about it before you meet them.
It's important to think of some questions to ask at the interview that are relevant. Ask about career options, how will you be recognised and valued, what are the targets or tasks you need to do and by when in order to be promoted. Certainly don't start by asking about holidays and sick pay!
Preparation is the key to success; so make sure you have done your research on the company, your potential role and why you would be perfect for the job. Remember that the purpose of an interview is not for the employer to impress you but for you to impress them.