Start with an existing poster layout. The Academic Technology Center (for printing) has a template.
Think in terms of "slides" - it is easiest to make slides, as for a presentation, and then place the slides in a poster layout.
Unlike in slides a presentation, you can have some content that is for reading (e.g., a full abstract). This is useful for when someone may browser your poster without you being there.
Have references (small and tucked away is fine) for appropriate works you pull information from.
Make sure you have content you can "point at" as you talk about your work and answer questions.
Step back and look at the overall aesthetics. Consider logos or other images that help with the overall look.
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