There are many code editor you can use for writing a react native app which includes VScode, Webstorm,Notepad++, Android studio e.t.c but for the sake of this documentation we will be using the VScode editor for screenshots .
Download VScode
Go here to download your VScode for mac or windows, https://code.visualstudio.com/download
and you can learn more about how to setup VScode here https://code.visualstudio.com/docs