Structured data, also known as Microdata, is descriptive metadata that is a function of your markup.
Because microdata is so closely tied to your markup, it is generally recommended that you update your theme manually to include microdata elements. There are some tutorials, and some "plugins" that will supposedly achieve this for you, but from experience (implementing on half a dozen sites) is that if you have any theme customizations at all you're really stuck with a manual effort.
An example of such customization would be as follows:
The Schema.org Doc: http://schema.org/Product
Open your template/catalog/product/view.phtml
div class=”product-essential”: Insert the itemtype
Some further reading: