Well, if you're printing it out yourself and photocopying it then Word would be OK. Seriously.
But...
Golden rules in this case would be:
• Impact and communication is the idea here
• One good image is worth more than two or more indifferent images
• Simplicity is key, but be bold with as few elements as possible
• Keep text to a minimum, 1-6 words large 48-72pt or more, then 50 words max for info, aim for legibility first, keep type effects to a minimum
A quick look at these vintage posters shows the same principles, which apply regardless of the era they were created in.
http://www.postergroup.com/browse.asp
Some of my favourite posters are the London Transport ones. You can see some examples here.
http://www.ltmuseumshop.co.uk/category.asp?cat_id=1&style=1
And in the case of photocopying, try and keep the tonal values contrasty rather than too subtle otherwise you'll lose detail.
Good luck!
🙂
(and don't be shy about posting your work)
Edit: But to answer your original question, if you're feeling ambitious then download one of Adobe's trials here
http://www.adobe.com/products/tryadobe/main.jsp
Photoshop may be your best bet if you keep pixel-resolution high or Illustrator. But if I was in your postion then I would use Word, putting type in boxes for ease of layout and playing with the letter spacing to get the type to look nice. Keep it simple.