How can I create a Microsoft Visio entity-relationship diagram from an existing Microsoft Access database?

Microsoft offers a simple tutorial on creating ERDs using Visio. You can find it here:
http://office.microsoft.com/en-us/visio/HP010181611033.aspx

However, if you are not familiar with concepts of ERD, I'd recommend you to study it first, and attempt to create an ERD on paper before using Visio. Directly using the software can add a layer of complexity!