1. Go to central admin --> Create or extend a new web application --> Create a new web application.
2. Fill the details such as Web app, DB and App pool name. Select yes to enable SSL on the web application. If you are using host headers for this web app, then enter those too. It is important to set the port to 443, not 80.
3. After the web application has been created, reset IIS and then open up IIS mmc. Scroll to the IIS website that MOSS just created for you and select the right SSL certificate from the available certificates. Go to the Home Directory tab and click Advanced. We need to set the host header and the right IP for port 80. For SSL entries, select port 443 and the IP. Click on the edit button for SSL entries and check the 'Require SSL' box. Also check 'Require 128 bit encryption' to make this more secure.4. Now we can create our first site collection for this web app. MOSS will automatically create a new site collection for us and present us with a "https://.." link upon completion. We are now have a SSL ready web app.
You may also like to read: How to enable SSL for selected pages in MOSS