My 2 bits. Start out writing your code for the Arduino. Once it is working, then start working on porting it to an attiny. Perhaps by staying with Arduino, and porting the arduino software to the attiny. Or perhaps by moving your code to avrstudio or winavr or gcc.
By starting with arduino you'll be able to start learning. Then learn a bit more. And then a bit more. Pretty soon you'll know enough to get where you really want to go.