Generally, you'll want your Nginx configuration to look something like the below to redirect HTTP to HTTPs:

server {
       listen         80;
       server_name    my.domain.com;
       return         301 https://$server_name$request_uri;
}

server {
       listen         443 ssl;
       server_name    my.domain.com;

       # The rest of your normal configuration
}

The default configuration file is located in /etc/nginx/sites-enabled/default

We might be able to give you more specific instructions if you could give more information like what you already tried or if you can describe exactly what you mean by "not working" (i.e. was there an error message).

Not sure how plesk handle nginx configuration, can be messy to mess around those files too if plesk overwrites them. but you can try this and add some speed/security to your site.

Open an account at cloudflare.com add your domain, and enable strict SSL this will route all your traffic to https even before it reaches your server, is free and add some security and speed to your site.

Now if you find the way via Plesk to avoid files being overwritten, then give it a try, but to be honest is a lot better if you avoid cpanels. One option I can think about it is to create 2 sites, one for http with a redirect (even a header redirect will work), and one for https.

Cloudflare will not make your site slower, if any will be faster, specially if you use spdy, which you should as you already have nginx up and running.

Go check cloudflare's blog, they have good explanations about the SSL and SPDY protocols they offer.