Please let me know the best way to expertise in Mainframes, also let me know how many divisions exist in Mainframes like JCL, COBOL, DB2..
One more Question: What is the scope of Mainframes in IT industry?
That's a lot of topics for one post! The best way to develop expertise in mainframes is to take the time. I recall one of my professors in college stating that studies proved it requires 4,000 hours to master a topic. So for mainframes and factoring in the 50% of the work day that you don't spend on the mainframe, you're looking at a minimum of 4 years of learning to master the subject. And that will be longer if you don't spend much of your time learning. Most of the experienced people on this and the other fora have more than 20 years experience with mainframes -- in my case, I started using mainframes in 1975 part-time and went full-time in 1977 so I have more than 39 years of daily interactions with mainframes.
I wouldn't call them "division" -- perhaps specialties would be a better word? For a broad classification, mainframe work can be divided into systems and applications, where systems focuses on the hardware, operating system, and installed programs (installing updates, making performance changes, doing capacity planning, and working with vendors on hardware and software upgrades) and applications focuses on the programs people use to get their jobs done (and there are a wide variety of applications -- financial, inventory, manufacturing, HR, to name a few). Anyway, depending upon how detailed you want to get, there are hundreds of programs that can be installed on a mainframe -- from programming languages to complete applications to skeletons that must be customized to the customer's needs.
I'm not sure what you mean by "scope of mainframes" -- if you use an ATM anywhere in the world, the last I heard there is a 90% chance that your data will be on a mainframe at some point. 92% of Fortune 500 companies have mainframes, and these days mainframes are used for pretty much any application you can think of. IBM is pushing the system z for cloud-based computing, for example. I wrote HTML code on a mainframe more than 10 years ago that was used to access -- from the PC browser -- manuals and help screens for the applications programmers.