you have 4 options...
Onsite training from peers
Internet
RTFM
Vendor driven Coures
Onsite training if feasable is best for learning inhouse processes etc
Best is to attend proper training courses for a specific Unix Flavour......
IBM AIX (admin 2)
Redhat Linux RHCE (certification)
Solaris 10 Admin
and so on....
if you a very adept studious person, the Internet and also Manuals/Books can also be a good way to familiarise yourself and get a grasp of unix.
"like trying to be a mechanic, anyone can tinker, but reading the book will get you through it, but nothing beats actual training or onsite training"