First of all gather all your information( only slogans and punching lines)
Gather many pictures from Internet.
Choose a nice colour for your poster.
Now with the help of a Glitter sheet cut out the heading of your topic in caps
Try to make shapes which will be suitable for your picture. Such as If you are showing an accident try to paste that picture in a zig zag shaped paper.
Don't forget to make border of your poster. It will let your poster look nice.