There are several methods of adding Media to your posts and pages and two sections in your Dashboard where you can do this. The first section being the Media tab on your left-hand Dashboard menu and the second allowing you to add media directly from your post or page edit screen using the icons found in the Upload/Insert group.
To view a step-by-step how-to guide on how to Insert an Image click here http://en.support.wordpress.com/links/image-links/ or view the following video:
If you wish to upload video files directly from your computer, you will need to purchase VideoPress.
View the “Embedding YouTube Videos into your WordPress.com posts” video
Using the VodPod video Widget:
If you wish to upload audio files directly from your computer, you will need to purchase the Space Upgrade. To link to audio elsewhere without storing them in your Media Library, check out other Audio options.
Adding Other Media:
Add Media (PDF, Microsoft Word document, etc.). This icon will allow you to insert a download link to a document or file. If you wish to embed the actual document/file into the post/page content, you can use Scribd.