Start here, you need to have a twitter account
Have a Twitter Strategy: from the inbound marketing pros at pr2020
Tweetdeck, Great program for accessing and using twitter from a Windows PC
Twitterholic.com, A fun site for finding the most popular twitter users.
Twitterberry, you guessed it, Twitter from your Blackberry
More Time Saving Twitter Tips from the inbound marking guru Paul Roetzer from PR 2020
A Comparison of twitter desktop tools.