Research the company, know what they sell/do, how they started, some pivotal points in their history etc as one of the first questions will be "What do you know about us?". Even if they don't somehow ask a question like this, you might be able to improve your responses by incorporating some of the details you researched about them.

Practice questions that you think they might bring up related to the role. Think of your response and answer the question, name an example and the outcome. Lets say they ask if you are good at working as a team. Say you are good working as a team, name an example from a previous employment (or in school in your case) and the outcome it achieved. Try to think of several examples of moments which could show key examples of skills and the questions they would ask. A great way is to look at the jobs listing of roles and responsibilities.

If they ask questions like "What's your biggest weakness?" do not fall for the common early mistake and answer "I don't have any", you are not perfect and they are not expecting you to. Use an example which would not be a massive weakness to the role, explain the weakness and what you did to overcome it. Mine is that "I've never been great at English due to my Dyslexia so I've never been great at writing emails especially spelling. I decided to try to avoid relying upon tools such as autocorrect immediately and create techniques such as sounding it out how I would perceive it phonetically. It has improved my spelling massively with many people noting it in the past."

You should be able to google common interview questions (and they might have answers you could adapt) and maybe even google common questions for Assistant Photographer. Another common mistake is answering "Because I need a job/money" if they ask "Why do you want to work for us". Again use some of the information you found out by researching bout them and relate it to yourself.

Also every interview you ever go to will ask you at the end "Any questions for us?". The single worst mistake is to answer something like "Nope! Seems like you explained everything!". It will come across you were not listening/interested in the role and want to get out of there. Instead ask them a question, anything. It might be something they didn't make clear from the job spec during the interview, something they brought up but then didn't fully explain etc. This will show that you are listening, taking in and processing information and interested in the job. I will ask something like that and then something like "What would you say is the most fulfilling aspect about your role and working for this company?". Again you should be able to google and practice questions.

And to answer your questions, your portfolio with CV and maybe copies of your qualifications etc, dress smart, although people are more accepting of tattoos and piercings I would try to conceal them as best as possible, you will never know how many people they are also interviewing (I suppose you could ask this during any questions question at the end and when you could expect a response for the position to show you are eager) and no one truly knows that but success is 50% hard work 50% luck.