This page is a good starting poin to find out what we have for interview preparations:
Interview Tips Webinar
First I would recommend to watch Interview Tips Webinar - It gives very practical tips and help you prepare better for an interview.
If you want a guide what is expected from a BI Developer then I recommend Should you become a BI Developer Webinar.
Interview mini courses and tests
We have mini coures with a set of pages that contains Interview questions and answers that cover: T-SQL, SSIS, SSRS and more.
To find it type "interview" in the filter by keyword box on our mini courses page.
T-SQL Tests are available from main menu (Practice // Tests) and cover beginner, Intermediate (Ranking functions only) and Advanced (70-461 Querying SQL Server Exam) questions.