First, let me say, uh, LINUX ON Z. Now, to the real question--how can anyone learn to use mainframes?. The same way anyone learns to use anything else in the computer world. Hit the library, buy a book (or just hang out in B&N for a few hours), search the net, get some example code...

That's how people learn, especially in the trendy world of Comp-Sci.

Geek1: "Have you tried [insert language du jour, such as Ruby on Rails]"
Geek2: (12 hours later) "Check out my new ap; it's in [language du jour]"

Don't have access to Z machine? Start with Hercules.

Here's a debug tip:

LGHI R1,X'DEADDEAD'