The free of charge Arm Cortex-A Series Programmer’s Guide for Armv8-A is an excellent starting point for those new to the architecture.
Arm offers introductory Arm Architecture on-demand training courses, including A-Profile and R-Profile architectures, and appropriate development tools.
Arm offers instructor led training for organizations wishing for deeper technical training, including specialized topics such as the Arm Neon .
Arm also has a number of Approved Training Partners who can provide training courses to meet your needs.