One of the simplest models you can getting started with is Cisco-IOS-XR-infra-infra-locale-cfg.yang. It allows you to set the device locale (country and language). You can find sample apps for this model at:
https://github.com/CiscoDevNet/ydk-py-samples/tree/master/samples/basic/ydk/models/infra
Make sure you read the README file for basic apps. It explains how apps are named and how to run them.
https://github.com/CiscoDevNet/ydk-py-samples/tree/master/samples/basic
Happy coding!
