You can get plugins for that: Twittercounter.com and Twitter Remote spring to mind. 
If you’re after a prominent logo I would use Twitter’s official logos from here:
http://twitter.com/about/resources/logos with a headline that says “follow me on Twitter”.
You could also amend the text above your official Twitter Widget to read Follow Me on Twitter.
I like the original Twitter widget from http://twitter.com/about/resources/widgets – looks nice, it’s very customisable and does the trick.
There’s also Twitter Tools (which is a plugin for WordPress and has some added functionality – but not the prettiest sidebar widget as it’s text based) http://wordpress.org/extend/plugins/twitter-tools/ or the one that comes as part of the JetPack plugin.