Surindaku that is because your nginx server is set up to serve the php files as download. You need to add a block that proxies requests relating to php to the php fpm process.
Please, if you have no idea how to set up a server either extensively research the topic (digital ocean has great articles on the basics) or find someone to do it for you. Getting a server to show a page isn't even about protecting your server against hacks, abuse etc.