Basic Need-To-Know When Creating Theme

I have WordPress installed on a LAMP stack, and I am ready to start customizing the interface. I am in the dark on where to start. Basically, I want to create a whole new look and feel for the blog, with custom css, graphics, etc. Could someone give me the rundown on the basic points I need to know when creating a new theme\template from scratch (or based on the default theme)? Especially, what files I should be concerned with. Thanks for your help!