Hello Neha, 

If you are aiming for ENGINEERING colleges like IITs then YES you must join a coaching academy but if your preference is top NITs then you can study at home because coaching institute will provide you enough materials for practice. If you are at most sincere and discipline with your work that you can study for 8 hours daily at home then no need for any coaching institutes. If you are a medical student then i Would strongly recommend you to join a coaching institute because there is no other option available other than that. 

I hope you will find my answer helpful. 

All the best