Embed code in your sidebar or body (Let's take the Facebook "like" button as an example.)
From your administration page select the "Design" tab->"Theme" and go to "Layout" under your current theme. Then, choose the page you want to edit. Drag and drop a text box ("my text block") into your sidebar or body!
Then go to developers.facebook.com/docs/reference/plugins/like/ to get the code for the Facebook "like" button and copy it.
Go back to your text block and click the little down arrow. Click on "Source", then paste all codes and click on the "Save" button.
The Facebook "like" button will appear on that page now.
From your administration page select the "Design" tab->"Theme"-> “Edit HTML/CSS”. From there, you can embed code anywhere you like(footers, headers, the body and the sidebar).
Embed code in your product description
From your administration page select the "Catalog" tab and then click on "Products". Then click "Edit" on the right side of the product you want to edit. Paste the code you get in the description part and click on the "Save" button.
To embed video clips from YouTube into your product description: go to YouTube and click on the share button of the video you want to embed and then click on embed. Copy the code you get into the description part of your product page (please note that the editing status of the description should be in source mode.)