This question has been asked by many people here. So here is a
guide to aid the downloading.
Make my points clear first cause this is a sensitive
topic.*Ahem*
USING THE BITTORRENT TECHNOLOGY IS LEGAL. ACT OF DOWNLOADING
TORRENTS CONTAINING UNAUTHORISED COPYRIGHT-ED MATERIALS IS
ILLEGAL.THIS THREAD DOES NOT ENCOURAGE YOU TO DOWNLOAD ILLEGAL
ITEMS. IT MERELY AIDED YOU IN HOW TO USE THIS LEGAL TECHNOLOGY. WE
DO NOT ENCOURAGE YOU TO DOWNLOAD COPYRIGHTED MATERIALS. IT IS YOUR
OWN RESPONSIBILITY IF YOU ARE CAUGHT FOR DOWNLOADING ILLEGAL
MATERIALS.
Examples of legal torrents include open office, game mods that are
big in file sizes.In fact if the author of the torrent publish his
own works in a torrent on his own accord, it is legal.
Ok i think i made myself clear enough 
So here are the steps:
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1) Download a download client so as to download your
torrents
2) Get your torrents from the author, or you may search for his/her
torrents posted on sites such as
Ok cant list too many sites here 
3)After you download the torrent file, open it with your download
client
4)Start downloading.
Thats all! Simple as that!
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From http://en.wikipedia.org/wiki/BitTorrent#Software
Authorized use of BitTorrent
A growing number of individuals and organizations are using
BitTorrent to distribute their own material. Many adopters report
that only by using BitTorrent technology, with its dramatically
reduced demands on networking hardware and bandwidth, could they
afford to distribute their files.
Software
Many major open source and free software projects encourage
BitTorrent as well as conventional downloads of their products to
increase availability and reduce load on their own servers.
Examples include OpenOffice.org[6] and most major Linux
distributions, including SUSE[7] and Ubuntu.[8] BitTorrent is also
used to distribute updates to the BitTorrent client itself, as well
as to other clients such as Azureus and BitComet.
Games
Various sites on the Internet like gameupdates.org offer authorized
game files via BitTorrent; the demo of the flight sim X-Plane is
offered via BitTorrent, as are the World of Warcraft ingame
patches. Another such example is PlaneShift, which uses BitTorrent
for its primary method of distribution. For the first time, the
demo of Football Manager 2007 is offered for download through
BitTorrent.
Film and video
The film studio Warner Brothers Entertainment plans to distribute
its films and TV shows using Bittorrent.[9] The fan-film Star Wars:
Revelations is distributing two DVD images as well as the film by
itself via BitTorrent, while Star Wreck: In the Pirkinning and
Cactuses, both feature-length films, were provided for download via
BitTorrent. The fan-films Star Trek: New Voyages are distributed
via BitTorrent among other methods.
mariposaHD is the first HDTV series made to be distributed over the
internet. The free Creative-Commons licensed videos are available
in both (1080p and 720p) HD formats. By utilizing Microsoft's WMV
HD video codec to compress the videos and BitTorrent to transfer
the files, mariposaHD are able to distribute data intensive high
definition video over the internet at relatively modest cost.
Music
The SXSW (South by South West) music festival in Austin, Texas has
released two packages of mp3 music filesnearly a thousand
tracksfrom their 2006 festival by BitTorrent download, along with
trailers to two DVD films that can be purchased.[10] Babyshambles,
Pete Doherty's band, distributes two collections of music, Shaking
and Withdrawn Megamix and Untitled by Bittorrent from their
official website.[11] In 2005, the rock group Harvey Danger began
distributing their third full-length album, Little by Little...,
using BitTorrent. Discipline Global Mobile (the record
label/website begun by Robert Fripp) uses BitTorrent technology to
distribute. Folk punk band Defiance Ohio distributes their music
from their website using mp3's, ogg's and torrents. The free music
portal Jamendo also uses BitTorrent to distribute its 1000+
albums.
Edit: Thanks davidche for pointing out. One responsibility off.

Edit 2: Thanks gigabyte for pointing out the error.