how would you get started on that kind of project?

a messaging app with a front end in React-Native and a backend in Go.

Use google. Type in google what you want to learn and read what it has to offer.

For react-native: https://www.google.com/search?client=firefox-b-d&q=react-native+tutorial -> https://www.tutorialspoint.com/react_native/index.htm https://reactnative.dev/docs/getting-started

For go: https://www.google.com/search?client=firefox-b-d&q=go+tutorial -> https://go.dev/doc/tutorial/getting-started https://go.dev/doc/tutorial/

Find similar applications and read their source code.

React native example frontend: https://www.google.com/search?client=firefox-b-d&q=react+native+messageing+app+github -> https://github.com/Ctere1/react-native-chat

Go backend example: https://www.google.com/search?q=go+web+backend+github -> https://github.com/amitshekhariitbhu/go-backend-clean-architecture