There are different ways of printing, depending on whether you are viewing a forum message/a content page or an attachment.
Printing a single message or a content page
1. Open the page you want to print in your browser.
2. Use your browser's print function to print (most often, browsers have a 'Print' option in the File menu).
Printing attachments
If the document you are looking at is an attachment that was launched in a different application (e.g. Adobe Acrobat Reader), you will need to use the printing functions of that application to print the document.