3 Jan 10
If you want to reply on a tweet or send them a public tweet all you have to do is type @username (username stand for the person's username) on the "what's happening area." putting it on the beginning of a tweet means that it's a reply to someone, if they are your followers to, it will appear in their wall(?) if not, they can check their mentions page.
6 Jan 10
That's right just put @ before their username and that's it. I advise you to try other twitter functions like retweet and reply. twitter is easier than facebook you need to familiarize your self and I assure you that you will enjoy it. Twitter is a friendly user social site. Goodluck!..