If you go to the BJCP site, you may tdownload a 74 page study guide, which I read through. They have a suggested reading list, too, but I did not look at that.
The questions were focused on: the style guidelines, brewing processes, diagnosing flavor flaws, and a few questions on the BJCP judging system.
There is a free, 20 question sample exam which must be completed in 6 minutes. The main exam is $10, contains 200 questions, and must be completed in 60 minutes.
After completing the 200 question online exam successfully, you are a 'provisional' judge and have a year to take the second part of the process, which is a tasting exam. Depending on your score on the tasting exam, you may be designated a 'recognized' or 'certified' judge. To be a certified judge also requires some experience points, which you accrue at competitions.