How and where to download Dota 2?

I am trying to download Dota 2 but am encountering a number of problems. Firstly, I am not quite sure which site is official and safe for downloading the game. In addition, I have heard that there might be different versions or requirements for installation, and that is a bit intimidating. Also, I am not sure what steps need to be taken to install the game without errors or viruses. I need help to understand this process and avoid mistakes.