Wow ... there are so many possible answers but I would definitely start with Javascript. Don't learn the whole language, just the "Good Parts". And that would be my first book recommendation - "Javascript: The Good Parts".

With that foundation, you could learn Coffeescript, NodeJS, JQuery, etc. Since you're talking about server side software, Javascript combined with NodeJS will give you both client-side (browser) and server-side capabilities.

You should take a look at the industries you want to join when you graduate to decide what back-end language you should learn. If you're working in a Fortune 500 company you're going to see lots of Java/J2EE. RoR is highly touted but not as wide-spread as you might guess. PHP and Perl are still widely used and there are popular Python frameworks as well.

Have a fun summer and good luck!