hmmmm maybe I can only send you a direct message if you follow me as well. I've just 'followed' you.
One of the first people you need to follow is MrTweet - in about 12-24hrs he will send you a Direct Message with a link where you go and he recommends 200 people to follow. I selected a few that I found interesting and some aussies to Network. A lot of them will then follow you.
There's a few help pages or tweeters here and there as well.
The main part is to post something interesting regularly and you will get people to follow you.
Don't flat out advertise too much because this will turn people off a bit as well. Helpful tips, link to a blog etc. is good.
Shorten and Track your links using:
http://tweetburner.com/
you don't have many characters so long URL's take up too many. With the track you can see your stats and what comments/URL's are being clicked on by the community.
Spend some time viewing the "everyone" link and follow those you find interesting.
have a look at some of my friends - quite a few are into marketing, design, helpful tweet tips etc.
a few more links I found useful:
http://mashable.com/2009/03/03/retweetability-index/
oh poo... I found other blogs, that give you 7 tips to tweeting and the like but I can't find them on this computer. I will have to check my other computer and get back to you on that one.
some applications that I haven't looked in to yet
http://twitter.pbwiki.com/Apps#GroupsandOrganizations
Basically I started by googling different things to do with twitter because I had trouble finding what I wanted to do on twitter itself. So definitely try googling

)
Hope some of that helps... Like I said, I'm still getting use to it myself.