Have they recommended that you buy and read a book? That would be the best place to start If they haven't you should think about investing in either 'The CELTA course' book by Scott Thornbury or 'How to teach english' by Jeremy Harmer and looking over the bits on language and grammar.
The more prep you do now, the less you will have to sweat during the course. The hardest bits for most people, except for the teaching , is getting to grips with grammar and parts of speech. This is stuff that you need to know to pass the course and be a good teacher, so starting learning it now wouldn't be a bad idea.
About the inteview, I might be a cynic, but they are running a business so they are highly unlikely to turn you away. These things tend to be diagnostic, if you do terrible at the language awareness they will tell you it's something you need to work on during the course.
Best of luck,