Click the Play store on the mobile.
Click the search button on the screen.
Type the name called whats app and click search.
Click the whats app icon on the screen.
Click the install button and click the agree on all conditions .
Download the app and the app will appear on the home screen.
I will show the screen shots of the following steps.
HOW TO USE WHATSAPP:
After installation open the WhatsApp.
Click the agree and continue .
Add your mobile to given the option and click go button.
After they send an OTP(one-time password) to your mobile number.
Then type your OTP in the verification block and click ok.
Then add your profile name and pic to your account.
Then invites your friends to your account.
If you want to download WhatsApp here click below: