Joining a coaching class depends on you.
you can prepare yourself and buy the online test series(offered by various coaching centres like CAT, Careerlauncher etc) for practicing.
If you really want to join a MBA/PGDM program right after BTech then do give 3 entrance exams-
CAT/XAT/MAT/CMAT/SNAP/NMAT/IIFT/ATMA
All the exams comprise of Aptitude, Data interpretation, reasoning, Verbal reasoning, GK
CAT doesn’t have any GK section.
MAT, XAT -GK score is not counted in the percentile calculation.
once you get your exam score. you need to apply for the colleges via their admissions page in the respective college’s website.
if you are eligible then you will be called for the next round: GD/PI/WAT