Styra DAS has over 100 Kubernetes Admission Control policies built right in, so you can get started in minutes—there's even a quick start to walk you through setting up your first policies if you like.
If you want to dive deeper and understand how to build fully custom policies with Open Policy Agent, then Styra Academy is what you're looking for!  We've built out 20 lessons on OPA, and its policy language Rego—directly from Styra, the folks who invented it!