Joomla offers only several options for SSL usage in the Global Configuration of the site. Under the “System” tab, you can set it to force SSL on all of the site, only the administrator portion, or none of the site. With a SSL certificate installed, I recommend having the administrator portion of the site selected.
In most cases, having the entire site using the SSL certificate is overkill. To force the SSL certificate on a certain page (i.e., a form submission page), go to the Menu Manager and find the menu item of the page you want to force SSL.
When editing the menu item, click “Parameters (System)” on the right, then switch “SSL Enabled” to “On”.