People often believe Graphic Design is like Art, a purely creative process. There is however a key difference. Art is created to enhance, to provoke an emotional response. Graphic Design is used to convey a message, it must therefor be clear, effective and adhere to a number of principles or rules in order to achieve that.
So what are these rules? Well lets start with No.1
Lets take the example of a movie poster. What is the most important piece of information you want to convey? Is it the title? The release date? The cast?
When planning the layout always distinguish between the most important and least important pieces of information. The easiest way to do that is to use different sizes of text. Position the most important info centrally, although it doesn't need to be in the middle.