I need help from you guys.

I will meet my potential supervisor two weeks later to discuss my project. He also told me that he will invite his colleague to have an interview with me. I think this is a formal interview which will probably affect my chance of being admitted. I am so anxious and sleepless now. Could anyone tell me what to prepare for the interview? If you guys have any experience like this, could you please share it with me? Thanks for your help!

There's lots of threads about interviews on here so have a little search. My advice would be to read up on the uni - student numbers, research interests etc. Also read the supervisors publications and recent literature surrounding your topic so that you sound like you have some prior knowledge. Always be positive, don't pretend you know the answer to something if you don't and be enthusiastic. Good luck!