Officially WhatsApp is not supported on tablets, but it works just as fine as on your smartphone. If your tablet has a 3G sim and associated phone number the method is straight forward. Download it from outside playstore because a search on playstore from your device won't show WhatsApp in the results just because it is not officially supported.
Make sure you have a messaging app for recieving SMS. Most tablets have a built-in one. If you dont have, you can download one from playstore like textPlus, TextFree, Textra, Pinger etc.
Temporarily change the security setting to allow the Android to install apps from unknown sources.
Go to Settings → Security → Unknown sources and check the box for "Allow installation of applications from both trusted and unknown sources". This will let you install an App outside playstore.
Download Whatsapp.apk from this link
Install Whatsapp apk file.
Whatsapp will ask your phone number for verification. Enter your number.
You will shortly receive the verification SMS with the activation code. Use the code to verify and you can start using WhatsApp.
Go on and start chatting with your friends and family...