Limewire is full of viruses, can be good if you know what you are doing; but, since you had to ask this question...
First you download a bit torrent client. Azureus or Utorrent will do. I prefer Utorrent because of its low memory usage.
Then you go to a bit torrent site like:
http://thepiratebay.org/
http://torrentvalley.com/
http://isohunt.com/
http://www.btmon.com/
Do a search for what you want and download the torrent. Now what you will be downloading is not the actual file but a file which tells the bit torrent client where to find the pieces of the real file.
Open the torrent file with your bit torrent client and it should begin downloading, sometimes immediately, sometimes it may take a minute or two and a download can be fast or slow, like several days even, depending on various factors.