Important basics of design for print - Use Adobe Illustrator (download 30 day free trial if you don't own it), and set the document size to the poster size, ensuring that all imported images are *at least 300dpi*. Generally for printing onto paper media you should set any colour mode to CMYK, otherwise expect to be looking at a very different colour to that which you intended for. Your chosen print house will love you for sending them a .ai file of the right dimensions and colour specification, all ready to go. You might have the option of 'encapsulation', which is basically a glossy, water resistant finish, otherwise, plain old matte will suffice.