if you are looking to pursue MBA degree right after your graduations then the best time is to start preparing for it and give exam plus carry out the rest procedure to get admission.To start preparations for MBA exams you need to first focus on mastering the basics of each section.

CAT is a test of how active, hardworking and techniques knowing you are.
To learn techniques you may join a coaching institute and try solving every type of questions. Don't go into detail, try variety of questions. Keep watch on the pattern of the exam.
Do mock papers and keep working.