Hello people,
I am new to hibernate OGM & mongodb concept, we have following requirement :
We have a project in which 10 Million or more users are expected to be registered within a year, and gradually they will increase.
We are planning to track user activities :
Any search they made ,
any record they like,
any record they share... and so on , there are various activites.

For the user activities part, we are planning to use hibernate ogm with mongodb implementation.
Currently, code is already developed using hibernate & mysql.

Can any one from this community drive me towards right direction, as to how & what data we should store in mongodb, what should be architecture, any precautions to take, etc...

I'm afraid there are too many variables in play to give you a helpful answer.

If you already have a a codebase that works with Hibernate ORM and MySQL you should be able to test it with Hibernate OGM relatively easily.
This will help you to check that all the JPA features you already used are covered by Hibernate OGM.

Good luck,
Davide