make sure you dress casual but comfortable, and look into these questions. they're very common...they wont ask every single one but you should expect at least 2-3
and like other people said it is very important to do research about the company, know the position you're applying for no matter how basic it is even if its just to get your foot in the door. and also have some knowledge concerning positions that are a little more advanced, or take more experience so you know what other opportunities they have. be honest, direct, firm. doesn't hurt to smile but don't be grinning from ear to ear the entire time. just be yourself and you will do fine. the nerves disappear once you finally sit down and start talking for sure