Download a .torrent
Open in your client.
Wait for download to finish.
Once finished, you can use your file(s).
Always remember to try to remain connectible, use uTP when possible, don't use a banned client, don't use Xunlei, don't use Xunlei, don't use Xunlei, seed as long as possible, open a port in your firewall *and* router, and only download content that you have the legal right to download.
EDIT: Oh, and before asking about speed issues, read the following guide very carefully and follow of its instructions to the letter:
http://forum.utorrent.com/viewtopic.php?id=15992
It's for uTorrent, but the BitTorrent client (mainline) and uTorrent are basically the same thing, just re-branded versions of each other, so any instructions for uTorrent will also apply to your mainline client.