Just a few highlights off the top of my head:
Get familiar with the following and use on a regular basis:
WRKSYSSTS – quick look at disk utilization, etc.
DSPJOBTBL – job tables can present a big problem if they fill up, so keep an eye on them. This is also a quick way to see how many jobs are allocated to spool files
PRTDSKINF – very good report detailing exactly what is consuming your disk
GO SECTOOLS – a menu with a wide variety of security reports that auditors really like
iSeries Navigator – this is really important if you want to pursue certification
Performance Tools – if you have them installed, definitely use them
I could probably come up with more, but these were just on-the-fly. The following websites are great sources of information, just to name a few:
From WRKSYSSTS, press F16 for Disk Status, F11 for more info
Use RT VDSKINF to collect info for PRTDSKINF. This may run several hours so schedule during downtime.
WRKPRB to display descriptions of system problems, both system-detected and user-perceived.
Found these old bookmarked links, still useful though:
At times, the most important thing to know is that you can move the cursor all over the screen and press <F1> to get a <help> display about the area the cursor is in. You ought to know this already, but don’t underestimate how <help> you can get. If you’ll be using new menus and screens, help for menu options and command keys can take you a long way.
The rest of the way should be found through the Information Center.