All web hosting providers will provide you free SSL soon. Right now, you can get free SSL from sslforfree website.
Verify that you own the domain through your web server or if your domain is not yet on a web server then verify it through the DNS. This prevents other people from getting an SSL certificate for your domain. By continuing you agree to the Lets Encrypt service agreement. You may need to whitelist 18.104.22.168 if your website is behind a firewall. If you receive a 504 Gateway timeout and cannot connect anymore then open another incognito/private browser or a different browser to connect again. If you have your own CSR use manual verification and input it after generating domain verification files. If you use IIS on Windows you may have to do additional steps.