To start development you need to initialise your project. You just need to run single command to initialise project.
After you executed below command "features" directory will be created inside your project, you need to place your features file inside this directory. Also, FeatureContext.php file will be created inside bootstrap folder, you need to create your step definition in FeatureContext.php file.
Run below command to intialise Behat
$ vendor/bin/behat --init
Note: If you installed Behat via PHAR, use php behat.phar instead of vendor/bin/behat for every Behat command.