Debenhams offer a massive range of roles, so do a little bit of research about what you will be expected to do in your duties so you can tailor your response to the questions to suit the position.
Dress professionally, and behave impeccably. Be prepared for questions about who you are, what you like, how you work in a team, and whether you have initiative to act on a scenario. Sometimes, interview with companies like this entails a small amount of time serving customers or handing out samples or leaflets to customers, so don’t be shy and carry this out to the best of your ability: it is a test of your capability to act on something unexpected!