I would review the Tableau Server guide on SSL configuration for the External SSL (connections to Tableau Desktop or Mobile... etc.)
The certificate will either be generated by an internal team, or obtained from a trusted authority, such as Verisign or GoDaddy. I am including an example for generating a key.
I hope this helps!
Thanks for your response! I have read the articles you provided. My server team asked if I need an internal or external certificate. The external certificate has a cost. Since we wanted to access Tableau using the mobile devices, I wasn't sure which one we need.