The recommended approach is to download the latest release of the pre-packaged farmOS distribution from Drupal.org.
The Drupal.org packaged release is pre-built and ready to go. Just drop it into a hosted web server environment and it will work the same as Drupal. For more information on installing Drupal, see the official Drupal Installation Guide.
Alternatively, you can also build the distribution yourself using Drush Make. This is essentially what the drupal.org automatic packaging script does, so it is generally not necessary to do this unless you have a specific reason to.
Simply checkout the [farmOS repository] and run the following command:
drush make build-farm.make farm
This will build the farmOS distribution in a directory called "farm". Point your server's webroot to this directory (or move the contents to your server's webroot) and open it in a browser to access farmOS.
You will need a web server with all the basic requirements of Drupal.
The only additional requirement is that you use PHP 5.3+. Drupal 7 itself only requires PHP 5.2+, but farmOS makes heavy use of the Openlayers module, which uses some newer features of PHP.