Use composer to install needed packages
composer install
Tu use the fixtures data, and try the site, you'll need to create the database, make the migrations and load fixtures.
First, we'll create the database :
doctrine:database:create
Then, we'll make migrations, so that we can create the tables, relations, etc ...
php bin/console make:migration
php bin/console doctrine:migrations:migrate
When the database and the tables are created, we can load the fixtures :
php bin/console doctrine:fixtures:load