i2c is right about getting some computers and connecting them up and looking at ways to attack and defend them. And keep in mind, they do not have to be brand new systems or anything. You can get Pentium and PII systems CHEAP these days and they can be great test dummy machice. Companies like RetroBox will selll you a decent PIII for around $60 (Needs monitor) eBay you can get PII's a dime a dozen. I picked up a couple of IBM PII/350/64/4g
systems for $19.95 each not to long ago to give to my kids school.
Either way, nothing compares to having experience, and building and attacking and defending your own network is a great way to learn. Or even have a friend try and attack it. etc.

One more thing. I strongly recommend you get the a good book on TCP/IP and one that tells you all the gritty details. No matter what part of computers you go into these days, you are going to need the a very strong knowledge of TCP/IP. Doesn't even matter what OS your are working with, being Mac, Linux or Windows. TCP/IP is a must!

Cheers!