download bittornado.
install bittornado.
bittornado should take over the .torrent file association.
Re-download the .torrent file and bittornado should automatically open it and ask you where to save it.
Tell it where to save it.
Wait a while.
Meanwhile, read http://dessent.net/btfaq/ -- the first link I found googling "bittorrent faq". It explains a lot about it. De-n00bify yourself.