how can i send a direct message to someone who is not following me?

You can't. You can, however, reply to one of their tweets and they will get the message in their replies tab. To do this, just type @username (where username is the name of the person you want to reply to) in the text box followed by the message you want to send. You can also reply directly to a tweet by clicking on the little return arrow thingie on the right side of a person's tweet (hard to explain, but you'll see it when you mouse over the left side of a tweet).

Be aware that these aren't private messages, though. Other people will be able to see them.

Hope that helps!