The true trick is to be prepared. Prepare a set of questions that you think they will ask and draft responses – not that you will simply repeat what you wrote but will practice the answers. Have a set of questions for them as well.
At the end of the interview, punch it – in other words, reiterate why you are qualified for the position and ask what the next step is – when should expect to hear from them. After, write a notecard of thanks!
There are plenty of websites on interviewing to learn possible questions that may be asked of you and ones you can ask the firm. Below is just one.
Be confident and remember that it is not always the best candidate that gets the position, but rather than one who knows how to job search the best.