[SmG FlasH™]

This is not really my 'homework' but something I'd like to know on behalf of my friend. 

So basically, I researched quite bit about Cross Site Scripting, and from my understanding its basically exploiting vulnerabilities in a web site/ application for one's own purpose (stealing data, etc).

My friend's question is 'How do you prevent XSS?' and 'What programming languages should one learn to prevent XSS?'. 

He's a networking student and he doesn't have much knowledge about programming, but he's looking to learn PHP/JS. 

Thanks in advance :)