It's hard to know exactly what they will ask you until you are in the interview. Be formal and polite, but overall be confident and stay calm. Do not get too nervous, if you do you may find yourself making stupid mistakes. Just give it your best shot. If you encounter a problem with something just take a deep breath and gather your thoughts. Should anything happen while online (Ex: internet access die and skype drops the call or you temporarily lose connection) apologize and explain the situation clearly and calmly. Do your best! You have worked hard to get where you are and deserve this chance so stay confident!