this is how to automatically get short urls for your blog posts in wordpress,

What you need
is to decide which tinyurl service you will be using and get its api URL,
for example rinyurl.com , bit,ly, tr.im,

here are the api URLS for the most famous services.

tinyurl.com : http://tinyurl.com/api-create.php?url=
is.gd : http://is.gd/api.php?longurl=
href.in : http://href.in/api.php?create=
tr.im : http://api.tr.im/api/trim_simple?url=

Note: There are some short url services api's which requires authentication, like bit.ly , so keep that in mind, but my above services are ok!