Hi.
- First create a Google/Gmail account if you do not already have one
- Then on your tablet's screen tap to launch the Google Play Store app
- Tap Existing (you may need to click OK first)
- Enter your newly created email address and tap Next
- Enter the password thereof and tap Next ... accept terms and conditions where prompted
- With the Play Store opened, tap in the search field and type 'Whatsapp' without the quotes
- Tap it from the results > download and install
Or follow the alternative method here