This seems to be the question of the day everyday.
It is in my opinion that learning any language takes time, obviously, but noone never seems to know what they want to learn.
Being young I would say if you up to the challenge and think you want to go into programming after High School and get a degree you will need to know either C/C++ and/or Java regardless.
Usually people who know C++ already have an easier time adapting to C since the syntax is very much the same, but not all.
These are probably the hardest languages to learn with the highest learning curve (in my opinion), but will give you a good idea of what is involved in not only learning but mastering a language. Once you master the basics of these the rest is easy, sort of.
For instance the first language I learned was C++, mainly becuase thats what they taught at my uni, but I learned everything I needed to be able to teach myself any other language. The verbage may be different but the sytax is the same. So Java, PHP, ASP, Perl, ect... weren't that hard to learn becuase I had that foundation C provides.
However, if you dont want to dive directly in start off with some simplier languages like VB or even HTML. They will at least give you a feel of what is expected. Even though VB is nothing like HTML they are both still considered languages.
Anyways my point is find a language that may be to your liking and work up or down from there becuase we all learn at a different pace and have different preferences. Noone can really tell you what youll like until you try it, we can just give you ideas of what to expect.
I do want to say that at 15 or whatever you are deffinately giving yourself and advantage becuase you will already know the basics of programming and especially if you get into Compute Science this will prove to be invaluable. When I was 15 I barely knew what a computer was more less programming languages.
There are numerous resources online for learning almost any programming language out there. Also do what I do sometimes. Sometime if you have a few hours to kill goto a book store and pick out several books on different languages. As you read through them you will quickly see which ones you may have interest in and how difficult they may or maynot be. You will see beginner and advanced books so be sure to start out easy and eventually figure it out. I wouldn't expect to be writing port sniffers or whatever after a week or even a month, this does take time, lots of time!
Im sorry for the book I wrote, but as a college grad I wish I would've had the forsite to better prepare myself and would like to help anyway possible. Remember programming and computer science isn't easy on any level and the sooner you get started the better off you'll be.

One quick note I would like to reiterate on what was said before. Get a Unix based OS like Redhat, Debian, FreeBSD or something. Not only having the knowledge to program on these (UNIX), but using them effectively will be real important. Not everyone uses Windows, Thank God!
Hope this helps,
Good Luck