Either click on the 'Open in DevPod' link above or start via devpod up github.com/loft-sh/devpod-kubebuilder-template. This will start a small devcontainer that has all the needed tools to start working on a custom Kubernetes controller. It will also start a small KinD cluster within the devcontainer that can be used for development.
After successfully starting the devcontainer, you can create a new KubeBuilder project:
kubebuilder init --domain my.domain --repo my.domain/guestbook
Afterwards, you can follow the KubeBuilder Quickstart to create a new api.