Find a template online or make your own! Just be sure it's created using PowerPoint (or the Mac equivalent) and that page parameters are set to 42"x36" or 36"x42". All files will be converted to PDF before being printed.
You can use this template to get your poster started. Feel free to get creative, though, and stray from the format that's here!
36"x42" Poster Template with Logo.ppt [Download]