Poster Conventions

Ensure you follow the poster conventions for your own artefact.
The major forms and conventions of film posters could include:

A large title which attracts the eye, bold colours which stand out
from the background, intriguing picture which would attract viewers to
the film, persuasive language which might persuade the viewers to go
and see the film, using the main character's name for promotion, and
the certification will either to ensure that the right type of people
are watching the film.

Try using either the rule of thirds or the triangle of interest as a format for your work. Have a look at this site http://www.colorpilot.com/comp_rules.html for more information about how to use the rule of thirds.

A successful film poster will accomplish attracting the target
audience and persuade them to come and watch the film