Wich strategy to implement testing in React-Redux app

I'm a french junior developper on react-redux and I just started a new job in a tiny company (5 persons, 2 developpers). I work with a senior developper and we are looking for testing practices to apply tests on our app. We already have so informations and we already know some technologies : Jest and React testing library ; and we would apply Test Driven Developpement in our futur projects. But we feel lost about the strategy to implement it, about unit test or integration test, about the time that we could save with test or not, about what we have to test in priority or not, etc. Open to discuss and I could explain more in details by email if you want. Thank you so much !