Catching up...
DR Jennifer - If you're Tweeting from Twitter's own web-interface (via Safari, Firefox, etc.):
-Bring up your "Profile"
-Click on the "time" stamp ("now", "two minutes ago", "22 hours ago", etc.) below the tweet you want to link to. That will bring up the tweet by itself.
-Copy the URL from the address bar in your browser.
If you use something like Echofon, Twitterrific, Tweetdeck, etc., you can just "right click" the tweet, and option to "copy link to this tweet" should pop up.