You'll need to go to Sun Microsystems' Java website and download the "Java Runtime Environment". It will probably prompt you whilst installing to install the appropriate Java plug-ins for your browser.
There is a lot of dicketry at ARFCOM, but there is also a mind-boggling proliferation of AR-15 information, ranging from newbie-question-and-answers to the latest and greatest from equipment and accessory makers.
~GnSx