Follow Laravel 5.5 installation instructions at Laravel Installation.
Change directory to root of application. Make a copy of .env file by running following command.
cp .env.example .env
Make necessary changes to .env file created above, i.e., database connection details need to be specified.
Finally, run the following commands in root of application folder.
composer install
php artisan key:generate
php artisan cache:clear
php artisan config:cache
php artisan migrate
php artisan db:seed