In order to download Guild Wars 2, first you need to buy the game, you can do so on the official website. At time of release and up until the end of December, the game was 60 dollars. This is subject to go down over time. [Guild Wars 2 main page]
You can find the page to buy Guild Wars 2 at buy.guildwars2.com/en or just buy.guildwars2.com.
This game is all the rage and sold over 2 million copies within the first two weeks of release. There is dynamic content that gets released periodically, so the game is never the same. Join the epic adventure as soon as you can!
And check out the "suggested by the author" links below for more!