Just like with any kind of poster you need to plan it well and then decide on the design you're going to use in order to convey your message. Yes, the message is crucial.
What is the message, then? Poster is a way of expressing an idea or presenting information in a more appealing way than text. This very idea or information is the message.
Knowing that, we can go to the details, you should:
1. Make sure that you know exactly what kind of message you will use.
2. Think about a slogan (here: Each One - Teach One should suffice)
3. Think aobut pictures which will support your message
(hint 1: choose pictures people can sympathize with better)
(hint 2: remember! People sympathize more if they can relate!)
4. Arrange everything on paper - remember: the more important - the bigger.
I hope it helps you :)
P.S. Examples in the attachments :)