You don't say what you want to do with the file, only that you do know what to
do next. I am wondering if you want to install BitTorrent. If this is the case,
you can install BT from the repositories. Go to System | Administration|
Synaptic Package Manager. Once there, do a search for BitTorrent.

However, Ubuntu already comes with a program to handle torrents. Hardy Heron
(8.04) uses Transmission. Other good torrent programs are Deluge and Azureus. If
you want BitTorrent then you can also get the GUI while you are in Synaptic,
otherwise you will handling torrents from the commandline.