Hi,
I have became PCB designer since 2012 and has done lot of projects, from Hobbyst, Robotic, Industrial, IoT, Audio, to Military grade application.
I know several common rule in PCB designing like For high speed digital communication, differential pair, Switching Power Supply, PCB antenna, power management system, isolated I/O, etc.
I used Eagle and Kicad, multiple layer is accepted, and I always refer to design guide provided by vendor.
Let me know what kind of PCB you want to build. my basic knowledge was Electronic Engineering, so I know how to analyze circuit.
Best Regards,
Rijal