You go to a torrent search site to find content you want. When you find the content, then you download a small file (the torrent) associated with the content. When the torrent is opened in a bittorrent client, transfer of the associated content begins.
Gizmo’s Freeware: Searching for Torrents (Popular Torrent Search Sites)
http://www.techsupportalert.com/content/searching-torrents.htm
Gizmo's Freeware: Best Free BitTorrent Client
http://www.techsupportalert.com/best-free-bittorrent-client.htm
You can also add torrent search sites to the BitTorrent search bar. This has info (uTorrent and BitTorrent are identical):
Gizmo’s Freeware: Adding Search Sites To µTorrent's Search Box
http://www.techsupportalert.com/content/additional-torrent-settings.htm#adding_search_sites