How to use Twitter's t.co service to shorten a URL
If you are tweeting from a "Tweet this" type button on a web page, you'll find the URL can be very long and takes up too many charcters. If you tweet within Twitter, t.co handles automatically shortens the URL for you. But not from external sites. Do this:
Prefix the URL to shorten with http://twitter.com/share?url= and then in the resulting page you can copy the URL from the edit box, or just Tweet from there.
Source: URL shorteners - How to use Twitter's t.co service to shorten a URL - Web Applications Stack Exchange