If you want to start from scratch, the short answer is always:
Take Quick Start courses. On the other hand, GCP has a variety of products and all of them have security involved, if you are starting, this means the recommended approach is to select 1 of them. Please refer to this
link for reference on a list of Security Products. Please go
here to access Google Cloud Skills Boost, or specifically the
Security Engineer Learning Path.
Couple of samples for Quickstarts:
Quickstart - Cloud IAM