First, search this forum for topics related to this as there have been many. Probably 5 or 6 in the last few months.
You will need to take two ground tests: the Fundamentals of Instruction (FOI) and the Flight Instructor Airplane (FIA).
To study for the FOI use
Aviation Instructor's Handbook.
The flight instructor airplane test is identical to the commercial knowledge test. Study that using the gleim and any other book you want to read up about that information.
Get the CFI oral exam guide and go through that cover to cover.
Now IMO the FOI is the most important work you have in this course. It unfortunately is not heavily pushed by many, which I find unfortunate. In that book it will tell you the levels of learning:
Rote: Memorization, read and regurgitate. Can learn things quickly but you forget them just as fast. Why cramming for tests makes many pass but they also can't remember a darn thing a week later.
Understanding: Being able to explain what you have learned in your own words.
Application: Using what you understand in your field of study. You are at this level when you can link understood ideas to their experiences in the aircraft.
Correlation: The ability to link ideas from one topic to the ideas of another. By far the hardest level to reach and when reached you will rarely forget that information. When you link words to a song beat you are correlating those words to a beat you hear. So correlation doesn't technically need to be with related topics.
Here are some basic tips and how to apply the above information to your form of study:
Notecards: To handle your rote memorization and understanding levels build notecards with definitions. Go through them daily for a few weeks and you will be set with the first level, but don't stop here.
Lessons Binder: To reach the application level you can start building your lesson binder. Pay particular attention to how you organize ideas using what you learned from the FOI to help. Also discuss the FOI information with any instructor as often as you can trying to link ideas to ways they can be applied to flight.
Correlation: Through the binder process, discussing, and analyzing on your own time you will begin to correlate various ideas. Nobody can bring you to this level, you have to bust your butt to get here. The more you discuss, talk, even argue, or just plain think about the ideas the more likely you are to come to your own conclusions and thus correlations.
Chair Flying: Something I love to have my students do and is a huge help for the CFI ticket. This is a simple task, think about being in the airplane, where you actually are is irrelevant (shower, car, bed, etc), and think about exactly how to do various procedures. Walk yourself through all the maneuvers you will have to teach and speak out loud as though you were teaching it. You might sound pretty dumb if you do this on the elevator up to the work office, but you might correlate an idea to a dirty look someone gave you too.
Final thoughts: The more you study and the harder you work at this the better off you will be, did I need to say that? But don't stop at the basic books, dig into the A&P book on engines, get a meteorology book, an aerodynamics book (Aerodynamics for the Naval Aviator is great), and participate in these forums/read past forums. There is a plethora of information on these forums on just about any topic you can come up with. The broader your knowledge when you go for this ride the easier it will be. You obviously don't need to know everything you read, but if you remember where you found it and can reference it quickly then you can use it to build the lesson you are asked to build for your ride.
Hope that helps, good luck.
PS: Get all the PTS books for whatever rating you are going for. In the case of just a CFI-A you need private, commercial, and flight instructor PTS books. Having the oral exam guides for each won't hurt either. Finally, a copy of 61-65E.
