I think it would be better if we design a poster rather than writing it.. :)
Anyways, here is the basic format for designing a poster :
=> the title should be catchy and creative.
=> slogans can be added to aid your content.
=> you can illustrate your poster to achieve a wider audience.
=> date, time, venue and the contact number should be mentioned in case if you are addressing an event.
=> the message should be clear and crispy
=> finally, the name of the issuer and their designation should be mentioned.