How can I sell a product in an interview?

I have an interview for sales advisor at a call centre tomorrow, my friend has already had an interview there and said you basically have to do a small test which is simple, then you have to sell a product in front of a group of people, I'm rather nervous as I have only ever had cleaning jobs, what do I start by saying? I think I will be talking on the phone to someone, my friend had to sell a Dyson hoover for example, please help