That is a tricky question, Systems Administration is a very broad topic. Can you give us some more information about the position you are in (or want). I assume based on the tags you are only interested in the Linux side.
The problem is that a sysadmins job is very tool based. I could tell you to learn about managing apache, but if you job uses lighthttpd or nginx then it wont help you much. I would read the index of "The Practice of System and Network Administration" by Tom Limoncelli and that will list the general topics better than i ever could. Here are some of the broad topics I have worked on (full time linux admin for years)
Web servers
-be comfortable with how to install configure and manage the webservers your company uses, apache, tomcat, nginx, lighty, websphere, etc
Email
-sendmail postfix qmail etc
-how to create strong policies and not expose yourself can be googled
Database
-be comfortable with basic sql
-know how to install and manage the database as well as basics of tuning the OS to support a database by changing open file limits and swappiness etc
Language compilers, VM's, linkers
-This one is tribal and changes anywhere you go. As long as you know how to install from source and ideally roll your own packages you should be able to handle this
-php ruby java python c c++ and a thousand others, again changes depending on the shop
Virtualization
-if it is used, take your pick of vmware, xen, kvm, etc
Backups
-Server backups, user desktop backups, database backups
SECURITY
-This one is unique in that is it less about tools and more about policy. Reading the sys admin books by Tom Limoncelli are a great start on how to create a policy, enforce it, and still maintain your sanity