Don't use it for following slebs, most of them are either mind-numbingly dull, completely vacuous or just ridiculously spoilt. Or Paris Hilton, who is all of the above.
I use it to keep up to date with what is happening in UK, to chat to MNetters and random folk that I have "met" there, and for a laugh when someone post something funny.
Good people to follow are journalists, who will interact with you, and will keep you well informed. And writers, bloggers, anyone in a field that interests you.
The disjointed conversations is helped when you use a Twitter app on your phone so you can see what the person replied to. And Tweetdeck is fav for the PC so you can organize the people you follow.
You can also search for any topic. So if you are interested in knitting, you have a search column "knit", if you like XFactor then search for that.
The @ is to address a person. So if you want to say something to me, write @mmelindor and I and my followers will see it in our timelines.
Use # to group messages. So during the election if I would use #GE2010. For the pope's visit there is the #popevisit. When everyone uses the same # it makes it easier to search for tweets about that activity or occasion.