First you need to download and install a Bittorrent client
(there are many others). Then go to suprnova.org and click on the link for the file that you want; it should open up the BT client and begin downloading. If the BT client does not automatically open when you click on the link, right click on the ling and save the .torrent file to your hard drive then manually open it with your BT client.
In my experience BT downloads can be very fast if you are connected to a number of peers and are uploading at a decent rate.
I recommend getting the Azureus BT client
, its the best one in my opinion.
Note: You MUST upload in order to get a good download speed. If you have DSL, I recommend uploading at at least 20KB/s.