I think we'd rather be burning your information

I need to produce a couple of posters next week, and as I've never done anything of the sort before, I would like your advice.

The purpose of the posters will be to advertise a meeting for a group of students, so it needs to be clear and informative, but not hugely complicated. Any recs on good free software to use? Advice on what works and what doesn't? Entertaining stories from past experience? All thoughts would be much appreciated.