Programming of larger microcontrollers becomes way more effective and convenient when using RTOS. It takes some time to get used to using schedulers in embedded systems. This is why good reference is always time saver. IF you keep an eye on ChibiOS/RT then there is a good starting point with STM32L-Discovery. Simply grab demonstration board, GCC compiler and keep playing around. Demo code has most of necessary tasks ready for you including initialization of clocks, I/O tests, DMA, ADC, SPI, PWM and serial interface. Once you’ve done with whats included you can use existing source as skeleton and start building your apps with no pain.