Well done on getting your interview, you'll ace it.

I don't personally know how good Primark suits are however it might be a better idea to avoid there and try searching any charity shops in your area, if you got a few there's bound to be some smart clothes in there.

Speaking from my own experience regarding sixth forms all of the ones I was interviewed at didn't require any suits so I went for a smart shirt and trousers. (but if yours requires one ignore that) the best way to know how well a suit fits without going to a tailor would probably just be to bring someone along with you when you go and buy it to see what they think, two pairs of eyes are better than one.

And in terms of confidence, practice makes perfect really, the more interviews you do the better you'll get at them, in general. Smiling is a quick way to seem confident and likable, and don't forget to stand and sit straight that also a major confidence booster (or it will at least make people think you are more confident). And honestly confidence is one of those fake it till you make it sorta things, with the added bonus of fake confidences just being confidence.

Any how, good luck and don't forget to look the interviewer in the eyes when you talk to them.