You have an up and running OpenShift Container Platform or Kubernetes cluster with Operator Lifecycle Manager installed.
Argo CD Operator (Helm) is published to both OperatorHub's for Kubernetes and OpenShift.
Visit OperatorHub.io and install the the operator as described after pressing the blue Install button. When the operator is up and running install Argo CD with the provided Custom Resource Definition under View YAML Example, choose one of these k8s examples or create your own customization with configuration values that fit your needs.
In OpenShift 4 navigate to Administrator - Operators - OperatorHub and install Argo CD Operator (Helm). When the operator is up and running install Argo CD with the provided Custom Resource Definition under Create Instance, choose one of these ocp examples or create your own customization with configuration values that fit your needs.
It's also possible to install the operator via one of our provided custom catalogs for Kubernetes and OpenShift. Please visit the Guides sections Operator Installation - Operator Lifecycle Manager of this documentation for your platform with more information about this installation methods.