Gosh, you are in a state!
I've interviewed many people for office work thirty years ago, and when I moved into Education as an aspiring TA BEEN interviewed several times. I ENJOYED school interviews, as it gave me the opportunity to talk about children and their learning - both things I enjoyed, and felt was an area in which I had something worthwhile to CONTRIBUTE.
If you enjoy working with children, know that they ENJOY you, and believe that you have something useful and worthwhile to offer in a school setting, then what on earth is there to be NERVOUS about? I'm a retired male TA, so don't feel too qualified as to what you should wear, but if you REALLY don't know that for yourself, then perhaps you shouldn't be trying for work in the first place. SMART & TASTEFUL, I would have thought: hopefully not piercings, tattoos etc, unless it's a freaky school.
Do you KNOW you are competent for the position? Have you worked in similar jobs before? Have you any SPECIAL experience or skills to bring to the job: arts & crafts, sport, music, languages, ICT etc?
I started as a 'parent helper', and one of my first Yr1 'readers' turned up in another school SEVENTEEN years later, on Teaching Practice in her final year of Teacher Training! Roles were reversed, as she needed to demonstrate she could direct me as a TA in the classroom. (She coped very well, in a slightly unusual situation, and became a successful teacher).
Relax; and I always feel I'm doing OK if I make the panel laugh a bit! (But don't overdo it).
GOOD LUCK!