How to make a rail application work over https?

Question

Nginx

I am not familiar with ssl certificates. What I tried to read - a dark forest. But you need to run in production on https. While I'm trying on a staging server. How to do?
staging.rb nginx.conf
config.force_ssl = true

server {
    listen 443;
    server_name m.y.i.p;

    ssl                        on;
    ssl_certificate            staging.crt;
    ssl_certificate_key        staging.key;
    ssl_session_timeout        5m;
    ssl_protocols              SSLv2 SSLv3 TLSv1;
    ssl_ciphers                HIGH:!aNULL:!MD5;
    ssl_prefer_server_ciphers  on;


    root /var/www/site/current/public;
    passenger_enabled on;
    passenger_env_var RAILS_ENV staging;

    
  }

sudo openssl req -new -nodes -keyout staging.key -out staging.csr
sudo openssl x509 -req -days 5036 -in staging.csr -signkey staging.key -out staging.crt
sudo nginx -t
nginx: configuration file /etc /nginx/nginx.conf test is successful

Running:
Your connection is not secure

Attackers may be trying to steal your data from the myip site (for example, passwords, messages, or bank card numbers). NET::ERR_CERT_AUTHORITY_INVALID

Did according to Ryan Bates
#357 Adding SSL

In a nutshell, more about certificates, if possible. Why are they paid and not free? It's still not clear what. Will all pages work using a secure protocol? Or is it possible to include all this only on some pages?