I am sorry to inform you that Whatsapp is not available for Nokia N73. Nokia N73 has Symbian OS and Whatsapp is compatible only with touch version of Symbian OS. As N73 is old phone it will not support most of the apps that are currently available in the market. As there are more users in Android, iOS and Windows; app developers are publishing apps that run on those platforms. However you can use apps like Nimbuzz, eBuddy in your phone.
It is not possible to download whatsapp on Nokia N73 phone. To make use of the famous whatsapp chat application, you may perhaps buy a cheap smartphone in exchange for your Nokia N73 phone. Any smartphone will be able to provide support for whatsapp application.
Nokia N73 is a very old phone and it has S60 3rd edition OS which is not supported by Whatsapp. Whatsapp is available only for android, windows, iphones, blackberry, symbian, nokia s40 phones. So, if you want to use whatsapp then buy another phone.