1. Go to Layout / Page Elements >> Add a Gadget >> HTML/Javascripts (if you don't know how to add a gadget, you can see addind gadget to Blogger)
2. Copy and paste this code into the content box:
<a href='http://twitter.com/USERNAME' title='Follow me on Twitter'><img alt='Follow me on Twitter' src='your twitter icon URL' /></a>
4. Save
There are some Twitter button examples here, you can use them if you'd like to.
https://lh5.googleusercontent.com/_qf2_jbv7m4k/TdtSbcSMaQI/AAAAAAAAAGM/Vp43oTj-1Cg/110835_27721_128_twitter_icon.png
http://www.805county.com/wp-content/uploads/2011/01/twitter-icon.png
You have learned how to add Twitter button to Blogger, and you may also like to learn how to add floating share button to blogger if you want drive even more traffic generated by your readers. Or just simply return to Blog On Blogspot to get the latest tips.