How to make call web service based on server to server authentication

I'm currently implementing SharePoint 2013 app communicating with an external web service. I plan to use server to server authentication to archive SSO.

I searched from google and find some documents talking about how to configure SharePoint server to server authentication. But I didn't find documents talking about how to get implement the logic to get access token and use it for communication. Is there any guideline on this? Thank you!