This post will be updated with more details later, but for some basics, check out these two links:

How-to guide on creating new posts in wordpress: http://codex.wordpress.org/Writing_Posts

A short video covering the same topic: http://wordpress.tv/2009/01/15/writing-and-publishing-a-post/

To insert media files into your post: http://en.support.wordpress.com/category/media/

To insert a gallery of images: http://en.support.wordpress.com/images/gallery/

 

Super simple example: To embed a youtube video, just insert the link to the youtube page, e.g. http://www.youtube.com/watch?v=hWUV9fyc4Rc and your video will magically appear: