In order to install and play a free trial of Word of Warcraft, first you need to make an account and have a free trial pass. To do that, you need to visit the following official website. After you completed the registration process, you will be able to download a client that will automatically download and install an appropriate version that is needed in order to connect to the official servers.
If you already have an account, and Wrath of the Lich King expansion installed, you can start the client and enter your login information. This will automatically update your game version to the latest one available. It should be mentioned that in there have been 2 others expansions released (Cataclysm and Mist of Pandaria).