In simple steps
1) You download a program to run on your PC. I use Azureus, but there are others. This manages the downloads.
http://azureus.sourceforge.net/
2) You find a site that offers things to download, such as www.thepiratebay.org and register yourself with them (there are other sites)
http://thepiratebay.org/
3) You search your torrent site for a file you want. Once found you click on the "download this file" option (or similar words)
4) This downloads a VERY small file which is the "torrent" file, and this contains info about the actual file to download.
5) You open your client program, like Azureus, then "open" the small torrent file you downloaded earlier.
(Or you can just double click on the small torrent file and it should open Azereus automatically)
6) Azureus then looks at this small torrent file, and with the information in it starts to download your movie or whatever you want to download.
7) After a time (if you are lucky) you will get the whole movie and you can then watch it.
The steps above are just an overview and it can take a while to get the hang of torrent files.