Hi. To download whatsapp on any android phone you need to first add a google account on your phone so that you will be able to use the google play store. Go to setting>accounts>google and add an account. If you have an existing account you can use it to login to your device or choose to create new if you don’t have any.
After creating a google account it will trigger the google play store to update itself before you can be able to use it, give it few minutes so it can complete the update process, it will use around 60m of your data. After the play store has finished updating, click the search button on the app and type “WhatsApp” and hit enter, on the results choose the WhatsApp messenger and tap “download”.