Common questions...
Why do you want the job.
Don't worry about saying that the immediate thing that appeals is the extra cash/flexible working hours or whatever it is that attracted you to this job. However, then back it up with some other stuff along the lines of I like working with people and that's why a job working for your company appeals to me.
What do you know about the company?
easy - research the company on the net and pull off some facts and figures
What do you know about the job?
You'll be doing something along the lines of: till work, stock replenishment and rotation (making sure the food with the shortest shelf life gets pulled to the front so that it sells first), customer service, both potentially working on the cust. serv. desk and also advising customers who need help, possibly light cleaning duties, stocktaking, etc.
What are your strengths and weaknesses.
Likely to get asked this one. Strengths - think of a few then think of examples to back up what you're saying. It's no good saying 'I use my initiative' if you can't then come up with an example of when you've used it. Weaknesses - slightly harder, but don't worry about telling them some. Mine might be I think very quickly so sometimes I might miss things, however I'm aware of this, so I take steps like writing lists to make sure that I don't forget to do something.
Dress smartly for the interview, shake the interviewers hand firmly, smile, and wait to be invited to sit down. There is something called the halo effect which means if you make a good impression in the first 30 secs the interviewer is likely to go easier on you.