System Administration Guide: Basic Administration - Sun Microsystems

I'd start by browsing through the manuals. You don't need to read line-by-line but get a good idea of what's available to you when you need it.

You might consider purchasing an Oreilly or two. Basic administration and TCPIP.

I'd browse these forums to get an idea of what other admins are dealing with.

Learn about the main applications running on your systems. tomcat jboss oracle.

Basically read all you can.