I would read up on your principles of flight as that is what a lot of the groundschool will be based around. Stuff like lift, stability, forces affecting an aircraft in flight, stalling and spinning. You will spin on the course and its good fun.
If you know what aircraft type you will be doing the course on might be worth a quick flick through the POH if you havent flown it for a while.
Try and read up on the lessons you will be doing ahead of them so you get the most out of it. There is a book written by R.D.Campbell called "The Flying Instructors Manual" which I would recommend.
It really is a good course, you will learn so much and have a lot of fun doing it.
I got a part time FI job about 24hrs after my FI rating was issued. Only thing with FI work is pay is not great. If you can afford to go full time do it. If not settle for part time like I have. Hours wise I have done about 75 in the last 4 months working at the weekends.