There are two differences, operating Kubernetes out there (planning resources, network, storage, CPU, quotas, RBAC) and developing applications in Kubernetes to get facts about running containers.
Kubernetes has one of the most mature documentation pages for an open-source system which makes it practical for newcomers to start operating the cluster and install/deploy the required software as complex container ecosystems. If you want to have a certificate in Kubernetes then you can have a look at the following Kubernetes Training Course.
In order to learn it properly(mater it) you can consider a simple roadmap to learn Kubernetes:
Run a Stateless Application Using a Deployment
Configure Liveness and Readiness Probes
Configure a Pod to Use a Volume for Storage
Using a Service to Expose Your App
Configure a Pod to Use a ConfigMap
Managing Compute Resources for Containers
Monitoring, Logging, and Debugging
You can choose any public cloud or your local machine. If you can go through this list and perform 50% of the activities listed without losing focus or motivation for a particular containerized application, you will be solving 95% of the challenges that a regular team faces every day when using a CO system like Kubernetes. The know more about Kubernetes you can have a look at the following video tutorial on Kubernetes. If you are more into reading then you can have a look at the following DevOps Tutorial which coves a lot of things regarding Kubernetes.