Not sure about a site, but this book is awesome. OK, there is a site, one of the authors is the creator of the Burp Suite assessment tool. (portswigger.net)

http://www.amazon.ca/Web-Application-Hackers-Handbook-Discovering/dp/0470170778/ref=sr_1_1?ie=UTF8&s=books&qid=1288494263&sr=8-1

Please Read the FAQs

I don't want to be the guy who just says read the FAQs, but this applies to your first question.

Languages for WebDev

How to Start Learning WebDev

https://www.reddit.com/r/webdev/wiki/faq

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Javascript is the language of the web; start with it. Learn HTML and CSS for styling. This will get you enough basic front end knowledge.

Resources for WebDev

• FreeCodeCamp is a good resource for learning web dev.
• I've heard good things about the Odin Project as well.
• More Resources for WebDev

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Pentesting

In order to be truly good at finding web vulnerabilites, you need to know the fundamentals of the web. Learning web dev is a good start, but you'll need to learn more indepth about how the web works. Also, there's more to pentesting than web. If you are looking into pentesting, you'll probably also want to look into other aspects of pentesting (exploiting binaries, finding vulns in mobile/desktop applications, etc.)

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Pentesting Resources:

• How To Learn Hacking <-- Watch this first
• Here's a nice overview of what you'll have to learn for pentesting

Two Must Reads for Web Pentesting:

• The Web Application Hacker's Handbook: Discovering and Exploiting Security Flaws
• The Tangled Web: A Guide to Securing Modern Web Applications

Also a nice youtube channel for learning .

Last but not least, use CTFs to practice your skills (and maybe win some prizes).

Videos about CTFs:

• https://www.youtube.com/watch?v=8ev9ZX9J45A
• https://www.youtube.com/watch?v=Lus7aNf2xDg

• How To Learn Hacking <-- Watch this first
• The Secret step-by-step Guide to learn Hacking
• Here's a nice overview of what you'll have to learn for pentesting

Two Must Reads for Web Pentesting:

• The Web Application Hacker's Handbook: Discovering and Exploiting Security Flaws
• The Tangled Web: A Guide to Securing Modern Web Applications

Also a nice youtube channel for learning .

Last but not least, use CTFs to practice your skills (and maybe win some prizes).

Videos about CTFs:

• https://www.youtube.com/watch?v=8ev9ZX9J45A
• https://www.youtube.com/watch?v=Lus7aNf2xDg