If you're just getting started with Twitter- I recommend you check out their rather comprehensive
Twitter 101 for Business Guide. It will at least step you through the basics.
There are of course, many other free guides out there about how to use Twitter, but it might be a good idea to check out the official one from Twitter which will also include the most up-to-date information.
It's also something that I plan to bookmark as a reference for any clients of mine who are just starting out with Twitter and need a primer.