You need a PC that will act as the server. This can be a physical PC or a virtual machine running inside Virtual PC, VMware Server, Hyper-V, ESXi, or whatever else is available.
If you want to learn server 2008 I would recommend that you buy the Microsoft "Self-paced Training Kit" for the MCITP:Server Administrator certification. It will come with several books that include education reading as well as labs and directions how to set up the networks for the laps. It's a good resource.
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CompTIA A+, Network+, Server+, Security+
MCTS:Windows 7
MCTS:Hyper-V
MCTS:System Center Virtual Machine Manager
MCTS:Windows Server 2008 R2, Server Virtualization
MCSE:Security 2003
MCITP:Enterprise Administrator