There is plenty advice on the web for writing posters. See the Resources section for useful links. Here is a very short summary of the key points to get you started.
Clear and short title readable from 1.5 meters away
Write a short and engaging introduction
Use no more than c600 words
Have a logical layout of 5 or 6 sections
Use graphics, images and pictures to explain your ideas
Use an existing template from the web if you can
Don’t use block capitals
Avoid elaborate fonts and dark backgrounds
Keep a simple colour pallet
Use good examples as models for your poster
Share your poster with interested colleagues to get comment and feedback
Include your contact information, your poster may be viewed when you are away