Creating a poster can be done in Photoshop, Illustrator, InDesign, or even in all three.
Today with paragraph and character styles there may be little reason to leave Photoshop in the first place. The steps of designing still remain the same in Photoshop, the difference is that you will not need to learn how to navigate through a lot of extra applications to create a fun and engaging poster.
Follow this step-by-step tutorial on how to make a poster in Photoshop!