create folder in your root directory assets/Fonts and add your custom fonts
you want.
create a file in root folder and called react-native.config.js
Add the following code to the file =>

module.exports = {
project: {
    ios: {},
    android: {},
},
assets: ['./assets/Fonts']
};


or 
module.exports = {
    assets: ['./assets/Fonts'],
  };


Then, run the following command in your terminal:
react-native link 

to use it declare this way in your styles:
fontFamily: 'your-font-name without extension'

If your font is Raleway-Bold.ttf then,
  fontFamily: 'Raleway-Bold'