does anyone have any tips?
Practice talking about yourself prior to the interview. What you say needs to roll off your tongue.
I would use practice questions (find common interview questions via Google) and prepare answers for them.
Be prepared that the interviewer might ask questions that might be unexpected. However, if you know your stuff and know how to articulate your ideas, then you should be fine.
Keep your answers focused on quantifiable results, and you need to be able to explain the situation, tasks, and what you did (STAR framework). Anyone can bluff their way through a question, but that means squat all if it's not quantifiable. For example, I could easily say that I am a brilliant writer in my spare time, but unless I have produced any writing or won any awards for my work I'm just talking out of my rear end.
As you don't have any experience, I would try to draw out experiences you had in your past e.g. did you work at a tuck shop? What did you do? When volunteering at a fair/charity, what did you do? It's not likely that M&S decided to invite you for an interview purely because you just went to X school and you got certain grades/qualifications. If you had some sort of experience in a customer facing situation, I would draw from it where possible.
I would also try to remember what you put on your application form/CV when you applied to M&S. They will be using your CV/application form when interviewing you and they will draw out more indepth answers from you based on what you put in your application/CV.
what kind of clothes should I wear?
Be smart. Ideally you shouldn't be more formal that the person interviewing you, but go in a suit if you don't know what to expect. At least go in a shirt/blouse, trousers, and formal shoes. If you are flying blind, go in a suit.
The idea is to keep things formal and professional, even if it's for a job that wouldn't require you to wear a suit or wear anything formal. It's more about being presentable to the person interviewing you and leaving a good impression.
the email said that they will ask me 3 things in-store that can improve customer experience, what should I keep an eye out for??
Put yourself in the shoes of a customer. What criticisms do you have about M&S catering's customer service?
Is it timely? Are the assistants knowledgeable? What other things do you want?
Your suggestions need to be practical, suitable for M&S's needs, and something relatively cheap e.g. not something like rolling out a red carpet for customers or getting assistants to fly to far away places to buy or pick rare ingredients for customers.
I haven't shopped in M&S in a long while and I am not you. Your suggestsions should be personal and you need to be able to justify why your suggestions are valid.