Journalist Simon Sharwood aimed this advice at public relations people, but the same basic principles apply to anyone new to Twitter.
Basic rules are don’t be selfish, don’t be evil.
Don’t lurk. Twitter has become very conversational. If you are listening, but not talking, you are not adding value to the social network and people will not value your input. In fact they’ll think you are a pathetic bandwagoning n00b, which will NOT be good for your reputation.
If you must lurk to get a feel for Twitter or to watch journos in the hope of learning something, do so by reading your intended friends’ tweets as RSS feeds before joining yourself. Consuming Twitter through feeds means you can do so anonymously, a good idea while you learn.