If you are reading this post with your iPhone or Android, just click on this link www.viber.com/dl and install the application. For Blackberry use the following link: viber.com/blackberry and for Windows Phone use viber.com/windowsphone url.
Otherwise, open the App Store on your iPhone, Market on your Android device, BlackBerry App World or Windows Phone Marketplace and search for Viber.
If you are an Android user and you can’t install Viber from the Android Market, it means that your phone (or tablet) is not officially supported. In that case, you can install Viber directly from here: http://helpme.viber.com/index.php/Knowledgebase/Article/View/100/16/cant-download-from-android-market
When you install it, it will prompt you to access your address book, and, after that, to enter your mobile phone number. When you enter the number, you’ll receive an SMS with an activation code. Enter it in Viber and that’s it!
Please note that, on Android based phones, in some cases, you will not have to enter the activation code manually. It will be automatically inserted into Viber.
If your device has no SIM card whatsoever (iPad, iPod Touch, Wi-Fi Only Samsung Galaxy Tab), you can enter the number from your mobile device and get the activation code from it.
Usually, new users are invited with an invitation SMS, sent by their friends.