Let me walk you through it:

    $ openssl genrsa -out $server.key 2048
    $ openssl req -new -sha256 -key $server.key -out $server.csr

Get the CSR to a CA and get the cert in your email inbox. Compose the cert into a chain (the most painful part of the process).

Put the cert and the key in /etc/ssl/certs/example.com.crt and respectively /etc/ssl/private/example.com.key;

In your nginx config add the following:

    server {
    
    listen 443 ssl spdy;
    server_name www.example.com
    
    ssl on;
    ssl_session_timeout 5m;
    ssl_session_cache shared:SSL:5m;
    
    ssl_protocols TLSv1 TLSv1.1 TLSv1.2;
    ssl_ciphers 'ECDHE-RSA-AES128-GCM-SHA256:ECDHE-ECDSA-AES128-GCM-SHA256:ECDHE-RSA-AES256-GCM-SHA384:ECDHE-ECDSA-AES256-GCM-SHA384:DHE-RSA-AES128-GCM-SHA256:DHE-DSS-AES128-GCM-SHA256:kEDH+AESGCM:ECDHE-RSA-AES128-SHA256:ECDHE-ECDSA-AES128-SHA256:ECDHE-RSA-AES128-SHA:ECDHE-ECDSA-AES128-SHA:ECDHE-RSA-AES256-SHA384:ECDHE-ECDSA-AES256-SHA384:ECDHE-RSA-AES256-SHA:ECDHE-ECDSA-AES256-SHA:DHE-RSA-AES128-SHA256:DHE-RSA-AES128-SHA:DHE-DSS-AES128-SHA256:DHE-RSA-AES256-SHA256:DHE-DSS-AES256-SHA:DHE-RSA-AES256-SHA:AES128-GCM-SHA256:AES256-GCM-SHA384:AES128-SHA256:AES256-SHA256:AES128-SHA:AES256-SHA:AES:CAMELLIA:DES-CBC3-SHA:!aNULL:!eNULL:!EXPORT:!DES:!RC4:!MD5:!PSK:!aECDH:!EDH-DSS-DES-CBC3-SHA:!EDH-RSA-DES-CBC3-SHA:!KRB5-DES-CBC3-SHA';
    ssl_prefer_server_ciphers on;
    
    # HSTS (ngx_http_headers_module is required) (15768000 seconds = 6 months)
    add_header Strict-Transport-Security max-age=15768000;
    
    resolver 8.8.8.8;
    
    ssl_certificate /etc/ssl/certs/example.com.crt;
    ssl_certificate_key /etc/ssl/private/example.com.key;
    
    #...
    }

Restart nginx. If you want to only do HTTPS, and have HTTP redirect to HTTPS:

    server {
        listen 80 default_server;
        server_name _;
        rewrite ^ https://www.example.com$request_uri permanent;
    }

It took 3 minutes to write this comment. Getting a new cert up and running will take you $5 and 15 minutes if you follow these instructions. Free if you use startssl.com. Cheapest wildcard I found was https://www.ssls.com/ when they ran a sale: $42. Current cheapest Google turned up was https://cheapsslsecurity.com/sslproducts/wildcardssl.html for $60. Personally, I prefer wildcard certs whenever possible, free certs from startssl.com.