Utorrent is my favourite program for torrenting. I prefer private sites like what.cd and waffles.fm to public ones. They're invite only though. ThePirateBay and MiniNova are good for public ones.
Speaking of which... I'd
love an invite to waffles.fm if anyone is offering...
op:
Basically you download a program (like any of the people who have listed here), install it and launch it, and then go find your torrent.
Personally, I just google '_______________ torrent' and try to find one with positive comments (some sites also have an option to 'flag' for viruses). I'm not TOO worried about viruses because I use a mac and I back up my files daily, but I've never had a problem anyway. Right, so, you search whatever, say, "Photoshop CS4 Mac Torrent" or "The Cars Best Of Torrent" or "Thumbalina Torrent" (or, "porn torrent") and then find a site, click the download link. It will download the file to your desktop.
Now, you open the program you downloaded before, and drag the torrent you just downloaded into the window. It goes to work. The progress bar should start working like any download. However, sometimes you find the torrent is dead (nobody is hosting, or 'sharing' it) and then you can either wait and hope or just go find another.
Good people 'seed,' or keep the torrent program running afterwards, so other people can download it from you - it's the only way to keep things going! Personally, I always seed a file for at least as long as it took me to download. I turn my computer off at night to save energy and I also use a lot of programs that use a lot of memory, so sometimes I've got to shut all my other programs down, but it's important to seed so that files can stay active.