HTML5 and CSS3 are the basic building blocks of websites. Though not technically programming languages, they're quite powerful and rather easy to learn; you could probably teach yourself a great deal in matter of months. Resources are plentiful, but Webmonkey
are a couple great sites to get you started. Go through the following Webmonkey tutorials in order to get a thorough grasp of these technologies:
PHP is another popular "true" programming language used throughout the Web, though a class would be helpful for learning this. PHP.net
is a handy reference once you know the basics.
As for mobile development, Android OS uses a version of Java, while iOS apps are built in Objective-C. There are also app-building services and programs available, such as the MIT App Inventor