By malmy at 14 February, 2013, 8:31 am
How do you make a tweet on Twitter only visible to people tagged? (Only people that are "@<username>) in your tweet? Thank you.
If you want to share something privately you should definitely not be using twitter.
But you can protect your tweets, which will apply to every tweet, here is how from the twitter website
How to protect your Tweets:
Go to your account settings by clicking on the gear icon at the top right of the page and selecting Settings from the drop down menu.
Scroll down to the Tweet privacy section and check the box next to Protect my Tweets.
Click the blue Save button at the bottom of the page. You will be prompted to enter your password to confirm the change.