How do I download WhatsApp on my iPad mini first generation?

The WhatApp developers have not released an App compatible with iPad - the App being currently limited to iPhone.

Whilst the App is not available for iPad - you should be able to access the service using a web browser:

https://web.whatsapp.com/

facebook does not make an official app for ipads

your best bet is like the web version

How to use WhatsApp on an iPad (trustedreviews.com)