Which programming language should I choose?

It's hard to completely "master" a language, but you should attempt to do so with at least one. Once you learn a language in depth, you have the ability to learn others. If you are interested in back-end development, I would suggest PHP or python. Go through the PHP track here at Treehouse, buy a good php book, etc.

The main key here is to never give up! Say you are going to choose PHP. Once you begin learning, keep going. Go as deep as possible into PHP, practice and practice. Once you have gone through some tutorials and book examples, try writing your own PHP application. I think this is a stage that all aspiring programmers go through, since it is almost overwhelming the amount of languages out there.

In my case, I learned JAVA at school. We went as far as getting into the basics of object oriented applications, didn't really get into GUI's or anything like that. However, I now know the basics of programming. My goal is to apply my JAVA knowledge to PHP and Javascript. I am going to learn Javascript first, because I want to start with the front-end side of things.

Good luck!

That's up to you! I'm choosing PHP, because most Content Management Systems are built in PHP(Wordpress, Drupal, etc.). If you want to develop your own Wordpress themes or Drupal themes, I would learn PHP.

My best advice would to focus more on the computer science aspect of programming vs which language you should learn. Being able to understand types, objects, variables, methods, classes, etc will go a long way towards learning any language. Syntax is going to differ among languages but those concepts typically remain the same or close enough so that you aren't relearning them very often. There are a lot of books out there that focus on all of these concepts, without going into language specific details.

With that being said, you still need to start somewhere and that somewhere really depends on what you plan on doing. PHP and/or Javascript are great places to start as others have said because they are used in so many places. There has been a huge resurgence of Javascript which is used on the web, natively on devices, and embedded systems too. It sounds like you really like working on the backend of systems so PHP, NodeJS, Ruby on Rails would all be good places for that type of work, Python is also a good choice.

I've personally hopped around with languages and have built extensive APIs, Angular based apps, WordPress commerce sites, intranets, but the one thing that was always a constant is the basic fundamentals of computer programming. All of those languages have been easier to pick up because of the prior knowledge I had.