How should i test in React Using the React Testing Library and Jest

I am building the FullStack Weather App with React and Express. I want to improve my app so I decided to add testing to my React. And it's my question, what should I test? I found that I should test things like events (click the button for example) async things, React router, Redux, Axios, what else should I test? I am super new in testing so for this reason I ask this question.

PS. I am using styled-components, and it changes something in testing or not? Have a nice day :)