What is it?
Try the Hello World, read the Developer Guide — in particular the Walkthrough and the other tutorials — and refer to the API Reference. Use App Templates as a foundation for your developments (available in SAP Web IDE or on github). Check out stackoverflow.com to discuss code-related problems and questions. You can use both tags, "sapui5" and "openui5". Or join the lively discussion in the SAP Community forum. You can also get an invitation to the unofficial OpenUI5 Slack channel created by the community.