What experience do you have with programming any microcontroller, let alone the STM32L family of devices?
I suspect that you are jumping in at the deep end without knowing the basics which is a path to frustration and failure.
What development environment do you have? If you do have experience in this area then I would start with the STM32Cube and the X-CUBE-BLE1 extension that can provide the drivers for the BlueNRG-MS devices. But again, I would not start here if you don't understand the Bluetooth technical principles - I've just been through that myself (many years of programming MCUs of many different types) and it is not trivial.
Susan