I would get a Teensy 3.2 and start programming bare metal with Arduino. You could also start with Arduino on AVR, but the Teensy gives you a lot more headroom a

I would get a Teensy 3.2 and start programming bare metal with Arduino. You could also start with Arduino on AVR, but the Teensy gives you a lot more headroom and performance and a very small physical size for very small $. Arduino is just g++ under the covers, and you can use as much or as little C++ as you want. You can stick to straight C if you like just by not using any C++ features.