cl@ude, I just want to give you some advise. If you want to learn about computer security I would ask just alittle bit diffrently. As you know you can't trust anybody on the net. If everybody here starts spewing off ways to break into a network or computer, some kiddies might take this knowledge as use it in the wrong way. So you can see how some people might think you are just tring to find ways to hack in to somebodys system and cause damage. I think the best ways to learn, first, get all the books you can on the topic you are looking for. If your wanting to learn about html and web hacking there are books out there just for this purpose. As well as any other platform your looking to learn about. Also this web site is one of the best tools you have. There are old threds here that can teach you anything you need to know. Some of the good books I like are Happy Hacker by Carolyn P Meinel. This is a very good book. It will teach you a ton of beginner stuff all the way to more advanced skills. also Network Security a hackers persoective by Ankit Fadia. This book is very in depth great desk reference. You mentioned that you know "C" you should also get a how-to book on perl a very good languge to learn. All of this will help defeat attacks agaist you machines and network. These books will give more of the information you are looking for than any collage networkoing book you may use. I would just sit back and read this web site for two or three weeks, get these books and study, and you will get what your looking for. Some people here don't like it when you ask how to hack something. I can understand that but do not be detoured from this site it is very helpfull
Hope I helped take care
P.S. A web sites www.Happyhacker.org also XTC46 mentioned Google. with out a doubt the most usefull thing you have to use. Check out the entire site they even have hacker in the language selection