React-Native - easy steps to create an App

react-native init YourProjectName

cd YourProjectName

react-native run-ios //for iOS

To reset the cache, do this below 

npm start -- --reset-cache 

Now trying to run on android, gave the below error 

A problem occurred configuring project ':react-native-reanimated'.

> SDK location not found. Define location with an ANDROID_SDK_ROOT environment variable or by setting the sdk.dir path in your project's local properties file at <path>.

this can be easily solved by creating a local.properties file and entering the 

sdk.dir = <Android SDK dir>

This may need to be full and not like ~/Library. Instead /Users/<username>/Library/Android/sdk 

Now meed tp set the android home directory also, which is like below. 

export ANDROID_HOME=~/Library/Android/sdk

Once this is set, good to go. it was running on both Android and iOS 

references:

https://stackoverflow.com/questions/44270504/react-native-ios-and-android-folders-not-present#:~:text=android%20and%20ios%20folder%20are,create%20android%20and%20ios%20folder.&text=This%20worked%20for%20me%3A%20Go,both%20environments%20%2D%20User%20and%20Workspace.