Sign up. Find people/groups to "follow" (ie. go to
http://www.twitter.com/Jeff_Head) and click "Follow" under the pic. Start posting comments in your "What are you doing Box". That is a "tweet", you can use commands (like @username message) to post to specific people, or like (#discussionname message) to post to discussions.
Go, go, go. It gets quitre addicting. Live feeds, unedited news from around the world. Use common sense and be careful and it is surprising how much you learn.