Customer portal is HelpDesk with more easy way to create new issues. It is enabled and configured by administrator. The main page looks like this:
Find what request type that you want to create by selecting groups on top groups menu. For example you want to report a bug and you don't know precisely what group to choose but obviously it isn't Sale so you can look for it in Support or Development if you are not sure. Or you have an idea of new feature and you've found an 'Idea' item in Development group. Just click on it to create new issue.
Fill in the fields and click 'Create':
You can see this issue in standard HelpDesk view by clicking on 'My issues' on header menu.
To create new issue refer to 'Customer portal' menu in the header.