The Nginx configuration for Ghost using the One-Click app is located at /etc/nginx/sites-enabled/ghost and looks like:

server {
    listen 80 default_server;
    listen [::]:80 default_server ipv6only=on;

    server_name my-ghost-blog.com; # Replace with your domain

    root /usr/share/nginx/html;
    index index.html index.htm;

    client_max_body_size 10G;

    location / {
        proxy_pass http://localhost:2368;
        proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
        proxy_set_header Host $http_host;
        proxy_set_header X-Forwarded-Proto $scheme;
        proxy_buffering off;
    } 
}

In order to serve a separate site on a sub-domain from the same droplet, you'd need to set up another server block. The key is to set the the server_name so that Nginx knows where to direct the requests. For a basic site, you'd simply need to add something like:

server {
    listen 80;
    listen [::]:80 ipv6only=on;

    root /var/www/html/mysite;
    index index.html index.htm;

    server_name subdomain.mysite.com;

    location / {
        try_files $uri $uri/ =404;
    }
}

Where root points to the directory containing your site.

For more info, check out the tutorial:

• How To Set Up Nginx Server Blocks (Virtual Hosts) on Ubuntu 14.04 LTS