What is a Secret in Kubernetes?
It is a ConfigMap with sensitive data that is encoded in Base64 text. What is a ConfigMap? It is a .yaml with a special format. There is always a "data:" section that is part of the YAML definition of a ConfigMap. The key-value pairs in the "data" section will have keys that appear in regular text; the values of the keys will be Base64-encoded. This encoding supports binary data as well as plain-text.
To create a Secret, you'll need to use openssl. This posting indirectly gives information on using openssl.
To create a Kubernetes secret, you may want to use Chapter 5 or page 216 from the book Kubernetes in Action by Luksa.
You may want to see the posting What is a ConfigMap in Kubernetes?.