Hii Gowthamireddy,

If you want to do the post graduation program after your Undergraduation,You will have to write the entrance exam for PG for that particular course.

To write that particular entrance examination you need to apply for it.you are allowed to apply for PG application form when you are in the final year of your graduation and can take the test in the final year itself .If you are doing BTech you can apply in the final year or if you are doing any 3 year degree course you can apply in  the 3rd year. 

Thank you.