The first thing what you will need is an account.
To register on HiPiHi in English, go to the homepage, and click on "sign up now", http://www.hipihi.com/en/
The second and last thing what you will need is the HiPiHi client. The recommended client is HiPiHi beta Dinosaur World 1.2, which can be downloaded on the homepage by clicking on the "HiPiHi World Installer" button. You can use this direct link DOWNLOAD NOW
More information about the two clients: Client compare