Congrats on your invite for the Interview. As far as studying for the big day, make sure you are current, IFR proficient, and up to speed on your basic regs and aircraft specs. If you want a basic list of areas to study and Part 121 regs you should be familiar with look at
http://airlinepilotmentor.com/technicalinterview.aspx
Dont study just to scrape by, get in the books and then you will feel really confident on the big day.

Good Luck