I get lots of questions about twitter when talking to clients.
The two questions I am asked most revolves around user names and hashtags.
My twitter user name is @adelaidetechguy.
Choose a user name that reflects you / your business / your personality
Sending a message starting with @twitterusername will give a public reply to the user of that name.
The other question is about hashtags (e.g.: #technology) which will categorise your tweets on a particular subject that will aid in other twitter users discovering similar and relevant posts.
Mashable have a great guide to Twitter