What are the topic comes under learning j-meter testing? like for database testing,login/logout testing,full project flow testing ,load testing of mobile api's
The topics are covered in JMeter User Manual in particular:
Also you can consider attending free JMeter Academy which is probably the fastest and the most efficient way of ramping up in JMeter tool.
The general advice is that your load test needs to represent real life usage of your application as close as possible, given each JMeter's virtual user represents a real user you should be good to go, otherwise your load test won't make a lot of sense.