First you need a bittorrent client. Vuze and Transmission are two good ones for Mac.
Gizmo’s Freeware: Best Free BitTorrent Client
http://www.techsupportalert.com/best-free-bittorrent-client.htm
Then 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
you go to bittorrent.com, download the program image file has extension .dmg, open it, drag the icon within the image to your applications folder, open the program, go to www.piratebay.org, search for something you want to download, download the torrent file to your computer, drag and drop the torrent file onto the bit torrent program, select where you want to save your download, the download will start.