how to use whatsapp

User Guide: Getting Started

http://www.whatsapp.com/faq/en/general/21073018

Welcome to WhatsApp! This is a multiplatform mobile phone messaging service that uses your phone's internet connection to chat with other WhatsApp users.
Before you download and install WhatsApp, make sure WhatsApp supports your phone type. Please check this here.
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. If they do not show up under the Favorites screen (Select contact screen on Android), carefully add their numbers to your phone contacts:

Enter the number the same as you would if you were to make a phone call to that person. Double check to make sure you entered the number correctly.
If this is an international phone number, do not use any exit codes or leading 0s. Start all international phone numbers with a + sign, followed by the country code. For specific examples and instructions, see this article.
Open WhatsApp and refresh your Favorites list (Select contact list on Android).

Need a more detailed explanation on adding friends? Read this article.
Looking for instructions specifically for your phone type? Learn how to add a contact on: Android | iPhone | BlackBerry | Nokia S40 | Nokia S60 | Windows Phone | BlackBerry 10.
Now that this contact is visible in WhatsApp, select their name and click on it. Now you can message your friend, and also send them media via the options in the chat screen. Happy chatting!

Additional information:
Provided you have an adequate data plan, it is free to send messages over WhatsApp. More details concerning cost are available here.
Cheers,
WhatsApp Support Team