OK, lets tackle your questions.
My fave and easiest to configure (also meets your one firewall port requirement) is uTorrent (http://www.utorrent.com/index-real.php ). Today, the guys at uTorrent have a sense of humor (April Fool's day) and the main page is a joke MPAA page, but they don't make judgements/know what you download. uTorrent is stable, fast, and easy. When you open it for the first time, say "yes" to associate .torrent files, and then the one time setup appears.
Follow the directions. Find your speed and utorrent will configure itself to torrent well. Next, you have the one firewall port. Utorrent picks one for you, and while software firewalls are easy, people often get poor speeds because of routers. If you have a router, pick yours from the list and follow the instructions:
http://www.portforward.com/english/applications/port_forwarding/Utorrent/Utorrentindex.htm
Once you follow the directions, hit "test". You should get a "green" result; port (#) is open to receive incoming connections. If not, make sure you picked the right port number.
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OK, next question. If you want to limit your upload to avoid getting a nastygram from your ISP, it's easy. The important thing to realize that your ability to upload directly impacts your ability to download. Basically, if you share with everyone else, all the bittorrent clients automatically figure out what a cool guy you are and work together to help you finish your download faster. That's my summary, but the basic premise is that Bittorrent clients see who shares and helps them the most. The easiest way to limit your bandwidth is to set a small limit and, as soon as your download is done (or shortly after), stop seeding.
I'll quote the uTorrent website (listed in sources):
"Does µTorrent have a scheduler to limit download and upload speeds?
Yes, in the options. It allows you to set an upload/download speed, and set it to Limit, go full speed, or stop torrents altogether by the hour/day. You can also make it turn off DHT during hours you set to "Turn off."
Version 1.4.1 beta 417 and later have a new scheduler mode for Seeding only. Hold shift while selecting the hours and it'll turn red, which indicates seed-only mode."
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The other questions:
-Bittorrent DOESN'T put any shares on your computer. The only file it shares, if it is open, is the file you have selected to download or have downloaded before (opting to "seed" it).
-All a ".torrent" file is is a tracker file for the file you're looking to download (the torrent file is a few KB usually). With that file, it knows what to look for, finds peers with the file, and downloads.
1) Use your preferred torrent search engine (downloading legally, I'll assume).
2) Find the torrent.
3) Download the .torrent file.
4) Open the .torrent file.
5) BT client opens, and in uTorrent if there is more than one file in a torrent, you can pick the files you want to download. Hit OK, starts downloading.
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Hope this helps!