Hello,
then which applications on mainframe should i focus on so that later on i can move to system programming
There is no particular application (i.e. accounting, order entry, customer service, distribution, banking, insurance, etc) that is better preparation for becomming a system programmer that i'm aware of. As people work in one appliation area or another, they tend to learn more about how the system really works. At some point, some of these people have learned the environment well enough to consider (and be considered for) moving out of the application area to the system programming area.
As far as working with assembler, i'd suggest talking with people in your locale (unless you are willing and able to relocate) about which employers have any assembler application requirements.