Photoshop is one of the many tools of the trade. I hear that Illustrator is better, something to do with how it handles vector graphics better - vector graphics are infinitely scalable, raster graphics aren't.
Please, take your time. There's no rush to create a deck. You're young, so take advantage of that - use the time to learn the tools, to refine the skills, to become a proper designer. Then you need to learn the business end of design work.
School isn't necessarily mandatory - I have a buddy who's a working graphic designer and an artist, mostly self-taught in learning how to use computers for design work. It took him a few years and a lot of self-motivation to get where he is, though.
Sure, there's a lot of people out there who don't learn any of this stuff, one way or another. They make some of the worst projects imaginable, too.
Meanwhile, this entire board is loaded with knowledge passed down from designers who came before you - LEARN FROM THEM. Read those topics and posts.