You must apply online, they will get in touch with you usually within 2 weeks whether you were successful for interview or not (don't worry if you are not invited the company keeps hold of information and some people get called for interview up to 6 months later), if you get interview great do a little research and interview can last upto 1 hour, dress smart and be yourself as although it is a job interview everybody is friendly and down to earth so relax and you will do fine.
Question 1 is about yourself
Question 2 is either what you know about the company or who are primarks competition and why
Question 3 why do you want to work at Primark
Question 4 what can you bring to Primark
Question 5 where do you see yourself in 5 years time
I am sure there are 1 or 2 more questions but very basic (I had the interview about 3 years ago with success). The main thing you need to remember is to be confident and open and you will do fine.