Hopefully it will help new beginners

Official doc here

If you dont have keystore than use before command else skip

Generating a signing key / Keystore file
You can generate a private signing key using keytool. On Windows keytool must be run from C:\Program Files\Java\jdkx.x.x_x\bin.

$ keytool -genkey -v -keystore my-release-key.keystore -alias my-key-alias -keyalg RSA -keysize 2048 -validity 10000

you will get a file like my-release-key.keystore

Setting up gradle variables
Place the my-release-key.keystore file under the android/app directory in your project folder.
Edit the file android/gradle.properties and add the following (replace ***** with the correct keystore password, alias and key password),
enableAapt2 set false is workaround , as android gradle version 3.0 problem

MYAPP_RELEASE_STORE_FILE=my-release-key.keystore
MYAPP_RELEASE_KEY_ALIAS=my-key-alias
MYAPP_RELEASE_STORE_PASSWORD=*****
MYAPP_RELEASE_KEY_PASSWORD=*****

android.enableAapt2=false

then add these app/buid.gradle (app)

below default config

signingConfigs {
        release {
            if (project.hasProperty('MYAPP_RELEASE_STORE_FILE')) {
                storeFile file(MYAPP_RELEASE_STORE_FILE)
                storePassword MYAPP_RELEASE_STORE_PASSWORD
                keyAlias MYAPP_RELEASE_KEY_ALIAS
                keyPassword MYAPP_RELEASE_KEY_PASSWORD
            }
        }

and Inside Build type release { }

 signingConfig signingConfigs.release

then simply run this command in android studio terminal
Below commands will automate above all answers

if windows

cd android
gradlew assembleRelease

if linux / mac

$ cd android
$ ./gradlew assembleRelease

if you got any error delete all build folder and run command

gradlew clean

than again

gradlew assembleRelease