To work with Drupal Behat Extension, you need to make sure few libraries and tools are already installed on your system.
PHP: PHP version must be higher than 5.3.5. This means you cannot use the same version of PHP for testing that you might use to run a Drupal 5 site. You can check version of php by typing below command:
$ php --version
if it is higher than 5.3.5, we are good.
PHP Libraries: You need these 3 PHP libraries: curl, mbstring and xml. you can check if modules are already installed by typing below command:
$ php -m $ curl --version
Java: Java will be required for Selenium. You can check if java already installed by typing below command:
$ java --version
Selenium Server: Download the Selenium server from here.
Also, make sure that you have site installed on your system or it is ready on some server, you can check the installation section for site installation using Acquia Dv