@elmusaf , in addition to the guide @rithvik shared, I’d recommend you check out the DHIS2 Development Academy workshops:
Web apps: https://www.youtube.com/playlist?list=PLo6Seh-066RzKN1UE_53LLEnhtPmy6zRc
Android apps: https://www.youtube.com/watch?v=9-3B8VJ7iD8&list=PLo6Seh-066RylaoOVDFYM_I5AZVUFFhoc