There's a book called Building Construction Illustrated that should get you started.

http://www.amazon.com/gp/aw/d/0470087811/ref=redir_mdp_mobile

pick up any francis ching book. they are wonderfully illustrated and quite helpful.

Francis Ching has some good reference books for a starter: Building construction illustrated and Architecture: Form Space and Order. It might be a good idea to regularly visit sites like ArchDaily to see what contemporary architects are doing. If you want to start learning design programs, try downloading SketchUp or Rhino (both have free versions). Good luck!

For a first project, this looks really good. Others have said some of the stuff that I'm going to say, but there's a couple other comments I haven't seen others make.

First, learn how to export images. Every arch program I've used has the ability to do this and it makes things look much nicer than taking a picture of a screen, which leads me to...

Learn how to Google efficiently. If you don't know how to do something, think of what you're trying to do, take the keywords out of it ("I want to build a wall that is sloped outward in Google Sketchup" turns into "slope wall sketchup"). In high school, I ended up knowing the programs we used better than my teacher because of this. Now in college, I am one of 3 out of about 125 that everyone goes to for help with programs. About 50% of the stuff they ask I don't know, but I can Google it and find an answer in under a minute.

Now, for the design itself. It's important that every design decision you make, you ask "why?" If you cannot fully justify it, think of a couple alternatives, and choose the best option. Then at the very least your reason would be "I explored a few options and determined this was the best solution." Sure, some will be able to argue it, but you have a reason. Always try and push it though. For instance, why did you choose wood planks for part of your facade? Is it because it looks good or because you had a location in mind and it matches the style of that location?

Next, materiality on facades. My general rule of thumb I use is one main material, one accent material (larger amounts of glazing would count, simple windows like you have would not). When you start having more than that, it starts to look rather busy and can be distracting. But like in the last paragraph, try to have a reason for the material. Pick a location for the building, learn the style and material of the location, and design with that in mind.

Finally, it's never too early to start learning about how buildings are actually constructed. If you can, get access to books by Francis Ching. If they are available at your library, check them out. If not, they're relatively cheap ($20-$30 each iirc). Building Construction Illustrated, Architecture: Form, Space, and Order, and A Visual Dictionary of Architecture are three books I highly recommend to get started on. It will help you understand how buildings are actually put together (and provide tips like nominal construction so you aren't doing things like cutting a CMU in half so that it fits). I notice a few things (such as being able to see the outlines of your stairs from the outside) that you want to watch for so they don't show up. This can be solved by understanding where different elements stop, how they're connected to each other, etc.

So, like I said, this looks really good. Starting at 15 is awesome, I started when I was 16 and now I'm applying to some of the top grad schools in the US, so definitely keep at it! One last tip, if you haven't already, start sketching/drawing by hand. It's an invaluable skill to have and will help you immensely if you decide to study architecture. Even if it's drawing one object a day, just spend 10-15 minutes every day sketching things out. You'll be surprised how much you improve just from practice in even a month.

For more technical, structural explanations and drawings Building Structures Illustrated and Building Construction Illustrated are both extremely helpful.

Do your grades for your diploma count towards UCAS points? If they do, then it shouldn't be too much of an issue as many universities go by that system and not strictly A Levels. What I would do is visit as many universities as possible, have a look at some of the work students do and ask the tutors. Some universities focus heavily on the technical side of architecture whereas others look towards a more artistic side. If you're getting a diploma on graphic design, then it would probably be best to lean towards the artistically oriented uni's as you might have a better chance with those.

Another thing you should do which is perhaps the most important thing is build up a strong portfolio and try to make it architecture related as possible. Many universities will ask for one and it can make a huge difference. I'd suggest buying an A3 sketchbook and a pencil and just go out and draw. It doesn't necessarily need to be a work of art, it's more about recording, anaylsing and communicating what you see around you. You could also buy a roll of tracing paper and draw some of your own designs over your sketches and photographs.

You could even start making physical models and using programs such as Sketchup to further develop your ideas and add them to your portfolio. If you really wanted to get technical, you could start looking into structure and detailing. Building Construction Illustrated by Francis Ching and subscribing to Detail Magazine are excellent for this and chances are you'll use them for life as well as throughout university.

It would also be worth looking at other portfolios to help you have an idea of what to include in yours and how to present them. I'd recommend big.dk as a good example, but be sure to look at others. Try to avoid too much writing, your work should speak for itself. When people are looking through portfolios, they're not going to bother reading lots of text so your work needs to stand out.

Good luck!

Architecture isn't a very textbook-y kind of field, so there aren't really many authoritative books that are universally used by everyone (other than the IBC).
Francis D. K. Ching is really the only exception to this rule as most schools seem to have incorporated at least one of his books into their curriculum. I would highly recommend the following:

Form Space and Order is a great introductory text that will introduce aspiring architects to some of the basic concepts of architecture.
Building Codes Illustrated relays code information graphically, making it easier to understand. It is not a replacement for the actual code, but it will at least give you an idea of what to look for.
Building Construction Illustrated is arguably the most useful of the three. I continue to refer back to this book even today and I'm not even in school anymore.

Mines a book: Building Construction Illustrated

I'm currently in graduate school. Most of the textbooks I bought were for my technical classes like environmental technology or structures. I have used:

Ten Books on Architecture by Vitruvius. I used this in my theory class. It's a pretty neat book that offers classic principles of architecture.

The Ethical Function of Architecture This is another theory book that offers more contemporary architectural issues. I'm not the biggest fan of theoretical readings but it's not too bad.

Building Construction Illustrated by Francis Ching

Mechanical and Electrical Equipment for Buildings

Fundamentals of Building Construction: Materials and Methods

101 Things I Learned in Architecture School This is one I think every architecture student should own. Its very small and simple.