1. While logged into your Twitter account, click Messages in the top menu bar.
The message window opens. From here you can send a private message to twitterers who are following you.
2. Click New Message.
The message window opens.
3. Enter the username of the person you want to send a message to.
Start typing a Twitter username. A list of possible names will pop up; when you see the name you want, click it.
4. Enter a message in the main box and click Send.
As with all Tweets, you must keep your message to 140 characters.