SSL must be setup on the web server and SSL port is 443 that is different for normal HTTP (80)
But, SSL is not just a port change, it is a different protocol and your web server must have all the capabilities to use it. SSL needs a security certificate issued by a third (You can generate your own, but it will not be trusted and warming messages will appear to users)
After your server is set up to receive SSL requests and map those HTTPS request to valid virtual folders (Can be the same as SSL, or you can choose to use different), all you need is something that switches those ports when needed
If you firewall supports that, then you are OK, but if not, my modules (MMSecureLogin or DSLocalizator) can help you to do that.
There are different scenarios, like a portal using SSL only for authentication or sensitive information forms. Or other portal with all functionality based on SSL
Remember that SSL takes more bandwidth and time to serve requests, so your decision could be guided by the power of your resources like your Internet bandwidth and your CPU speed
My modules are there only to help people accomplishing tasks, Their use is not mandatory at all.