You need to appear the entrance exam like IMU CET conducted by Indian Maritime University, JEE Advance, CUSAT CAT etc. for Merchant Navy. You can check the eligibility of the exams & Syllabus with given below links.

IMU CET Eligibility

CUSAT CAT Eligibility

JEE Advance Eligibility

Salary varies according to the designation & your skill as well but at the junior level you can expect the salary 40 to 50K per month.

Top institute for Merchant Navy you can prefer:

Marine Engineering and Research Institute, MERI Kolkata
Marine Engineering and Research Institute, MERI Mumbai
Lal Bahadur Shastri College of Advanced Maritime Studies and Research, Mumbai
T.S. Chanakya, Mumbai
National Maritime Academy, Chennai
Indian Institute of Port Management, Kolkata

Good Luck!