Hi. The exact steps largely depends on the AG Mobile model you have (the model number/name is normally written on a white sticker under the battery). Otherwise, you can normally download Whatsapp by: Opening the phone's internet browser > type: www.whatsapp.com/download and click/tap 'Go'. You can kindly comment below with your model number/name should you still require assistance.
Signature: SA tech Q&A is a free question and answer site. If the answer/solution provided above does not help' please use the reply button below to request further assistance.