Once your SSL certificate is installed Joomla 3 can be told to make use of it. Joomla 3 SSL (https://) for secure transactions can be setup globally or per Menu Item.
Fast way - Globally
In Joomla Administrator go to System -> Global Configuration [Server]
Set Force SSL to Entire Site.
Now all pages http requests will be redirected to https.
Setting Per Menu Item
In the Meta Data tab of a Menu Item there is the Secure setting.
The default setting is Ignore.
On will use https.
Off will use http.
You will need to set the secure links to On and links to non-secure pages to Off. Leaving links set to Ignore could cause unpredictable results. This is not hard to set up for a few Menu Items,but could take a while if you have a lot.
