want good topic for DBMS project

First of all Java is frontend and MongoDB is backend. Languages are always frontend and DB's are always backend.

As you said, you don't have much idea of doing projects, you can do whatever is in your mind. Some starting point can be good old Library Management, School Management, Attendance Management, Hospital Management application.

And finally regarding how to proceed, do you know Java? In that case I don't have much to show you how to proceed, but if you don't then learn Java first.