How to write a React component without using classes, only using functions?

Hi. I’m new in React Native and Expo, this is my project: GitHub - santiagoglobal/project_register: CRUD Expo React Native
I need put a calculate function in a textInput from Registro.js. Some tutorials show code that I cannot apply in React.Component classes because I don’t have functions but classes. I get the following error:

Error: Invalid hook call. Hooks can only be called inside of the body of a function component.

this is for put the const within the render(){ } or out of the class.

I need change the classes React.Component for functions, some examples?