B.E.

Hello Nagarajan, It is very easy to crack competitive exams until 1. you are confident and follow a proper day schedule that includes schedule of study, sleep, eat, exercise etc. For the productive study, it is very important to do exercise daily that keeps your mind sharp. Proper diet is another important factor to keep you healthy that will help to focuses on study better.
2. Besides all these factors, to crack a particular entrance exam you must have the basic knowledge of all the topics related to your exams.
3. You must try to solve all the previous year papers. It really helps a lot.

Post Graduate Diploma in Computer Applications PGDCA is designed for graduate students who are interested in computer applications. This course has been made for students who want to learn computer applications in different fields like banking, insurance, and accounting. The eligibility criteria of JNU Jaipur is given below for an idea. Eligibility : Graduation in any stream with a minimum of 48% marks
Selection Procedure: Merit + Interview
So from above you can be eligible for the PGDCA course after your completion of your B.E.

Yours 12th board exam are very important and give top priority to them. BBA or other entrance exams are the later on. More than 50% of entrance exams syllabus is coming from the 12th subjects so you can easily prepare the remaining syllabus of entrance after the 12th exam. So give yours 100% to your 12th board exam.

Hello Vaibhav Daga, I am afraid that you will have to wait for at least one more year. Students who are already graduate or are in the final year of graduation are allowed to appear for CAT. Best wishes to you.