That's a pretty big question.
Dreamweaver is without any doubt one of the most powerfull WYSIWYG editors out there right now. However to really get the most out of it it would probably be best to know a bit about HTML and CSS and maybe even PHP if you want to do anything too powerful.
http://www.w3schools.com/ is a good place to start. After that you can start learning more about Dreamweaver itself.
Free Dreamweaver 8 video tutorials from video-tutes.com has a few good tutorials that should get you going.
Best of luck to you with Dreamweaver!