Unless it's something very creative, your look needs to be as neutral as possible. You want them to notice you, not your clothes. The clothes musn't distract attention away from how great what you're telling them is!
As long as your outfit is fairly classic and plain (wear your best neutrals!), you need to focus on making sure you look groomed and put-together - everything clean and in good condition (no holes or stains) and ironed. Spend time doing your hair and wear subtle, natural-looking make-up. Discreet jewellery. A good, clean bag (if it's leather, polish it with neutral shoe polish) and an umbrella if it's raining.
When I had my interview for university (mature student, studying medicine) I wore a grey/black polka dot silk skirt (floaty/skater style, knee-length) with a smart grey suit jacket, black opaques and black slightly sparkly ballet flats. I'd had my ankle taken out of plaster the previous day, so couldn't do heels! I took a black patent doctor-style handbag. No coat as it way May and quite hot.
I honestly don't think any of the jury (10 people) noticed my outfit at all. And their own outfits ranged from smart businesswoman-like to mad professor
I have my uni interview on Monday and I'm wearing a high round neck wrap dress which is black with navy leopard print (it's not very noticable print tbh) with black tights and black patent riding boots and matching bag. It's sensible yet have a small hint of 'me' in the print.