If you've just downloaded the code, congratulations!!
To get it working, follow these steps:
Download Composer dependencies
Make sure you have Composer installed and then run:
composer install
You may alternatively need to run php composer.phar install, depending
on how you installed Composer.
Start the built-in web server
You can use Nginx or Apache, but the built-in web server works great:
php -S 127.0.0.1:8000 -t public
Now check out the site at http://localhost:8000