There have been different URL shortening services which could be used to shorten your links and then paste them in your tweet field. However, Twitter has now introduced its own link shortening service t.co.
The service is automated. When you paste the link in the Tweet field, the t.co service shortens it to 23 characters. There is no concern as to how long the original URL is. But, the fact remains that even if your URL is lesser than 23 characters long, it will, nevertheless be treated as 23. There is no option you can opt out of the t.co service. Twitter uses it as a means of finding how many times the link was clicked. It takes care of security too. If a site is a risky one, you will receive an alert.
Link shortening service
You can use the URL shortening services like bit.ly. You can enter the original URL on those sites and shorten it. This shortened link can then be posted in your tweet. However, if you are shortening URL for using it on Twitter alone, it may not be fruitful. The t.co service will still consider it as 23 characters long.
Live....and Let Live!