Welcome to WhatsApp! This is a multiplatform mobile phone messaging service that uses your phone's Internet connection to chat with and call other WhatsApp users.
To download WhatsApp, visit this link on your phone. Once installed, ensure that the friends who you wish to message have WhatsApp installed on their phones.
To start chatting, tap on the new chat icon and select a contact's name.
Provided you have an adequate data plan, it is free to send messages over WhatsApp. More details concerning cost are available here.
WhatsApp Support Team