Hello Kapil, please check out the following link for detailed info on Merchant Navy career options:

https://university.careers360.com/articles/merchant-navy

Also check out this official link for an institute offering Fleet management courses since there arent many: https://www.iifmuniversity.com/

You can take up diploma or post graduate diploma courses in Fleet management. You can apply online to the institute directly for admission, the eligibility is that you must have passed class 12 from a recognised board with more than 50-55% aggregate for joining diploma in FM, and/or a bachelors or diploma degree for Post graduate diploma in FM.

Best of luck