You can find WhatsApp on the Play Store of your phone. If you have another store, you can use it because it will install the official application, but if you don't have an automated way to install this application, then visit its official website. Download the APK file and transfer it to your phone using the USB cable. Go to Menu > Settings > Privacy and enable the option that allows you to install unsigned application then use the built-in file manager to browse to file to install it.
