While having a formal background in graphic design is absolutely going to be helpful, it’s also relatively easy to learn the foundations of graphic design on your own. An increasingly easy-to-use Adobe Illustrator and even more easily accessible tools like Stencil and Visme are making it so that just about anyone with two opposable thumbs, a bit of creativity, and motivation can earn a side income doing things like designing (and selling) images like these motivational quotes that can be printed onto posters and sold on platforms like Etsy. Or you can find a local startup, small business owner, or photographer who could benefit from some extra help designing or altering images.
But before you can graduate from side business idea and start earning a full-time living as a graphic designer, you'll need to build your skills—I recommend starting with reading the foundational book Graphic Design School and Steal Like an Artist, the incredible book by Austin Kleon about how to become more creative. To accelerate your education in becoming a graphic designer even quicker, check out the online courses Graphic Design Fundamentals and The Graphic Design Bootcamp. Then once you're an expert at your craft, you can further your education and move up to offering more hands-on experiences like design sprints for higher-value clients around the world.