Look at the job description and think of some examples of your work (student work counts if you don't have much experience). You will likely be asked to describe your roll in creating that work. The more detail the better. Did you work on a team with a review process? Did you engage in meetings with management or the client? Was your work expected to be pixel perfect or did it involve a high level of knowledge of Photoshop or some ability in front-end code? What was the broader implications of the work you did? Was it technologically advanced or require a fast turn around time? Did you upload your work directly and if so how, or did you coordinate with a programing team?
The placement agency is basically looking to see whether they can sell you to the client. Be honest about your level of experience and skill. They aren't expecting perfection, but if you have any experience at all with certain applications don't be shy about expressing that. Generally these interviews are pretty casual. They wouldn't call you if they didn't already see something they were interested in. They will ask about how much money you want up front. Be honest and don't shortchange yourself.