There are usually some useful layouts in whatever software you are using - Microsoft Publisher or whatever. I presume you are doing it on a computer, not as nice as real poster art but a lot easier to design and to reproduce at a copy shop (if you want the posters to be bigger than your printer can do).

Otherwise, make sure the vital information (what, where, who, when, how much) is easy to read and understand, and give a contact email or 'phone number for the information you can't include.