Not knowing what you already know I would start to learn following things:

- get used to the shell and basic Unix commands like
ps, df, ls, cat, more, mount, ifconfig, netstat, rm, touch, find, date, etc.
- AIX has superb man pages with examples and also a good documentation ie. the IBM info center
- get used to smit(ty); btw with F6 or escape+6 you can always check what command smit is going to fire off behind the scenes
- text processing & shell scripting (ksh, grep, tr, sed, awk)
- LVM (lspv, lsvg, lslv, ...)
- what the ODM is and how you can query it etc.
- package management with smit or installp, lslpp, rpm, ...
- backups with mksysb
- read about virtualization (HMC, LPAR/DLPAR)
- NIM
- start reading about performance tuning (vmstat, sar, svmon, ...)
- how VMM works
- keep reading AIX forums to find interessting stuff
- also consult IBM red books

Most important is:
-> Try out everything you can without risking the production.
-> Always ask if you don't know or if you are unsure.