Step 1. Choose from the buttons that Twitter has prepared on their website. A variety of buttons can be seen at http://twitter.com/goodies/buttons
Step 2. Once you’ve chosen the button you’d like to use on your site, click on it and fill out the button options form that will show up beneath it.
Step 3. Copy the code on the part that says “PREVIEW AND CODE” and then paste the code on to the part of the html source of your site where you intend it to be.