Crossplatform UI Work

[Turophile]

G'day all,

I hope that you might forgive this terribly "newbie" question...

I have had a look around and I cannot actually find anything so I am at the point of asking.

I wish to put together a simple application, mostly for personal use for now, but I would like to look into the future and supply some cross-platform compatibility. As a BSD user I have issues with many licenses (yes, I'm one of them, if you must) so I am interesting in learning how I could achieve something like this...

I am not terribly apt with C/C++ but I am more than willing to learn (I should have a fair bot of time after November through to March and I would like to keep thinking in it).

Basically what I want to know is:
a) How are things like, say, Blackbox rendering things on X11?
b) Is it easy to port this to, say, Windows, or is it almost a complete re-write?
c) Where can I find information on how to do this, more or less, from scratch (or as needs be, from a BSD license)?

Any knowledge on how this is done regarding X11 or how I might do it for a specific platform (BSD, Linux and Solaris I imagine should be similar if not the same; Windows; Mac) would be greatly appreciated.

Regards,
Alex.