Webapps security advice

If you wanted to learn web applications from a security angle, where would you start? I.e. you won’t be ever developing software yourself, but you may be asked to audit it, pen–test it at a later date.

With that in mind are there elements of web development that you don’t need to focus so far on, or are you best just learning web development from the angle of a developer, and take the security angle out the equation.

And with so many languages, where do you start? I.e. how do you identify which language to learn, and is it easier to transfer to another language, or do you need to learn each from scratch? Is there a “most commonly used” language out there, or could you give me in perhaps 1-5 with 1 being the most commonly used, 5 being a rare development language just so I can see what’s most “out there”.