You have to find some information is related to movie . ( photo , the name of the movie festival , ) Idea that related to it also ( film , camera, photo , the shape of prize .........) then try to find out which object you like to put inside the poster , then find out which composition is good . Make sure all the things you choose is related . ( For example , use a video camera , the lens show different kind of movie , cartoon ,friction comedy .........)
The most important thing in doing 2D design is to show 3D image and clear information show to the people who is reading you poster .
Don't use some Art words that normal people don't understand . Because poster is to show for all people including children and elderly