If you wish to become a registered WinRAR user, all you need to do is make a payment to the author, via one of the regional dealer sites listed here. Many dealers will accept credit cards, all will accept cash or cashier's cheque.
Please see the file order.htm in the WinRAR distributive for details.
You may visit WinRAR home page for buying online or to read the latest sites list.
Upon receipt of your fee with the completed registration form you will receive a registration key file which will correspond to the registration string given in the registration form.
Upon receipt of your registration key file you should put it to WinRAR folder or to %APPDATA%\WinRAR folder. If the key is archived, in a .RAR file, please extract the key from the archive in order to register your copy of WinRAR.
It is possible, depending upon the facilities available at your local registration site, to have the key delivered to you by e-mail or by your local postal service on diskette.