Write to your audience. Think carefully about the language you use and the assumptions you make about the viewers' knowledge. Don't use technical jargon unless you're sure it will be familiar to your audience. An indication of how familiar you are with your subject is evidenced by how well you can convey it. Keep in mind also that your poster will be competing with others for attention.
Graphics can include maps, charts, diagrams, illustrations or photographs.
You may need to modify the size after you work out your content. Also think about whether you want it in landscape or portrait format. Generally, you will not have room to devote more than one or two paragraphs to each of the above. You will also want to include acknowledgements and perhaps information on sources. A slightly different way to organize your information would be to follow the problem statement with the conclusion.