hi,
here are some tips for you: first of all dress properly, take a copy of your cv with you, speak politely,don't smoke, be ready to answer questions related to your cv, think about what you are going to say if they ask you something about yourself, think about why are you the person suited for the job you apply for.Try to find out as many things as you can about that charity shop and set up in advance the questions you want to ask the employer.
This is a site where you can find plenty of information about jobs and interviews:
http://www.cvtips.com/interview_questions_cont ent.html