Go to this page: http://twitter.com/goodies/tweetbutton
to create your code to embed the button.
Copy that code
Go back to Blogger > Design > Edit HTML. Check the box "Expand Widgets Template".
Search the code for this:
Paste the code AFTER this line. If you paste it before, the button appears in between the entries.
Find a suitable position on your HTML code by experimenting: cut and paste the code into certain segments of the HTML code, like after "Post Labels", "Author" etc. to obtain an optimal position for your tweet button.
Apparently you can also customize messages. Got the know-how from this page: