Try to in/exhale before the interview, present good posture and don't be worried/nervous to boost up your confidence! I know it may not seem like much for you but it helped for me atleast. edit: and most important of all, DRESS NICELY!
Just be yourself man, make eye contact whilst answering the questions, don't be afraid to take some time to present a more collective idea of what you are going to say. Shake the interviewers hand when you first meet them, have positive body language and smile to show you're interested. Try not to fiddle your fingers or fidget in your chair. Answer all the questions truthfully. Try to prepare for some questions, such as "why do you want this job" (do not say money under any circumstance) and so on. Try googling the common questions asked when interviewing for a retail assistant. Engage in off-topic conversation with the interviewer if you deem it to be necessary (before or after the interview) even if it's just asking if they're alright and so on. Remember you will be compared to previous applicants and future ones for this job so you want to stand out. To do this at the highest success, you will want to accommodate several things interviewers are looking out for: confidence throughout the whole situation; fluency in your speech; and likability as a person in general. Come off as someone who is genuinely interested in the job. I would indefinitely recommend researching about the company you are applying for as to not come off as unprepared - being prepared for the interview is one of the most beneficial things you can do as a candidate. I'm assuming you've applied for the same job at a few different stores. You should identify your priority reason for taking the job. This would be something like the salary, how enjoyable it is to work there, or location of the job (how close it is to where you live). If you are unsure of anything, don't be afraid to ask the interviewer, in fact, asking questions after the interview has finished actually stands you out to be more enthusiastic towards the job. Some questions you can ask at the end of the interview are "what is the best way to travel here from (where you are)", "is there an opportunity to work overtime", "how would you describe the ideal candidate" and "when will I be contacted about the job". Don't sporadically force a question at the end of the interview, and don't feel as if you have to do it. Only ask a question if you want to know the answer. Lastly, good luck man! I'm sure you'll do more than enough to get the job. Project your confidence and gleam in the room!