There is no prerequisites to the OCMJEA certification. However,
you should some design experience with
JEE.
The cert consists of 4 parts: MC exam, assignment, essay & mandatory training. The training can be done anytime.
I suggest you go thru the exam objectives of the MC exam 1Z0-807 and get yourself familiar with the topics. You may want to use the training to brush up your skills for the exam.
As for the assignment & essay, Oracle will give you a case study of some sort and you have 6 months to complete, producing a bunch of UML diagrams and files describing your design process etc.
This cert does require quite a bit of research and reading for your preparation depending on your background.