I agree with all of the advice above.
As an assistant/employee you'll be representing your employer so make sure you are presentable. Always dress better for an interview than what would be normal for the job. Have your references prepared and forewarned. Make sure that any written correspondence or material you bring is well written and error free. Show up early and well groomed, if there is a receptionist introduce yourself and ask for the washroom so that you can freshen up. Try to get a good night's sleep before. If you have your own transportation it might be an asset but it must be presentable. Have all your supporting documents with you: drivers license, S.I.N., proof of residency and current address, certificates-dilomas-degrees, awards and letters of recognition, etc...
Be prepared to answer some personal questions regarding heath and dependancy issues, be frank about your availability regarding evening and weekend work. Do not answer the question, if asked, about the rate of pay you expect, best to be diplomatic with that one. Answer honestly if asked why you left your last place of employment, even if you were fired or if was just as a part time bag boy at Walmart. Get as much information as you can about the firm before you get there, shows you are interested in who you'll be working for. Good chance you'll be asked why you want to work there and what your career aspirations are so think about your answer and be ready.
Try not be too nervous, look them in the eye when answering questions, don't be overly friendly at first - show some respect. Don't sit down until indicated to do so. Use polite and appropriate language, avoid slangs and acronyms. Try to remember to say 'photographer', 'photography' and 'photograph' instead of 'tog', 'shoot' and 'pic'.
Good luck.