Reddit mentions of Hacking Exposed 7: Network Security Secrets and Solutions

Start with learning computer systems, networking, and Linux. You need to be able to at least read computer code, know how data flows between computer networks, and how to do things in Linux. Here are few links to get you started:

First and foremost, basics and free stuff:

Intro to Linux
https://www.edx.org/course/introduction-linux-linuxfoundationx-lfs101x-2

Computer Networks
https://www.coursera.org/course/comnetworks

Intro to computer science and programming Python:
https://www.edx.org/course/introduction-computer-science-mitx-6-00-1x-0

Web development -- Will help you when (and if) you go through web pentest route
https://www.udacity.com/course/cs253

Cryptography
https://www.coursera.org/course/crypto


Once you've covered all above topic, you are ready to enter into pure-hacking learning:

First free stuff:
http://www.reddit.com/r/HowToHack
http://www.breakthesecurity.com/p/hacking-tutorials-for-beginners.html
http://www.securitytube.net/

Following cost money but take you through each and every step of a pentest without distractions:

Hacking Exposed ed.7
http://www.amazon.com/Hacking-Exposed-Network-Security-Solutions/dp/0071780289

The Hacker Playbook
http://www.amazon.com/The-Hacker-Playbook-Practical-Penetration/dp/1494932636

Very expansive but well worth it (Bonus: It's a certification):
http://www.offensive-security.com/information-security-certifications/oscp-offensive-security-certified-professional/

Hacking Exposed should be a good place to start. I used this book while I was going through my attacks and countermeasures class in college, and it seemed decent. Also, check out the /r/netsec and its sidebar. The only other advice I can give you is to get written permission from the businesses before you start doing any testing.

You mean the seventh edition? Yeah, you're right. I read the fifth one (2005!), and I can tell some methods are now deprecated or have increasingly evolved. But I don't know about 2012... However, I was still able to learn a lot from it. And it taught me to find resources to keep learning, so, it definitely wasn't a waste of my time.