There's already plenty of information(Questions/Posts) in ServerFault for a beginner Linux administrator:
- Linux How-to/Tutorial sitesLinux How-to/Tutorial sites
- Setting-up Linux at homeSetting-up Linux at home
- Tools you should absolutely know as a Windows/Linux AdminTools you should absolutely know as a Windows/Linux Admin
- Good Linux Podcasts/Links for beginnersGood Linux Podcasts/Links for beginners
- Hidden Features in LinuxHidden Features in Linux
- How to recruit a Linux GuruHow to recruit a Linux Guru
- What a beginner should know about a SysAdmin JobWhat a beginner should know about a SysAdmin Job
In addition, I will suggest you find an old computer and install Linux on it. Play with it, setup a firewall, DNS, DHCP on it to start and test things. You could even set it up in a VM and do stuff in it. Nothing is going to help you better than getting yourself in there for the real experience. Decide of a good use for it and try to use it as much as you can. Ask questions when you get stuck (research it first, though), that's why we're here for.