The easiest way to get started is to just write a driver for whatever simple device you have. It may be easier if you do some electronics as a hobby since you

The easiest way to get started is to just write a driver for whatever simple device you have. It may be easier if you do some electronics as a hobby since you can create the devices yourself with simple enough protocols. It gives you a good understanding of how the hardware works at a basic level. There are thousands of tutorials out there on how to get started writing simple drivers, as well as the excellent book "Linux device drivers". I don't do _complex_ drivers, but I've been able to write simple ones for my DIY electronic devices (keyboards, sensors, ...) and even a reverse engineered one for my BOSS RC5 guitar pedal.