Alright, I was cruising google and found this nice little article. It's a good primer on NETBIOS and some of its security holes. More importantly it is an easy concept for newbies to grasp, and can be practiced on your own windows machine. Or even better, try it on your home LAN if you have one. Thid should go without saying, but I will say it anyway. Don't do anything stupid, don't forget about a little thing called IP Logging if you do something stupid, you will most likely get caught.

http://www.mycgiserver.com/~ethicalhackers/netbios.html