Hello,

I hope you are doing fine, Before you understand computers, you need to understand design, so start with looking up the principles of design and the elements of design. No matter what medium you use (photography, hand cut lettering, collage, or computer-aided images), you’ll always need to understand these in order to do well.

First prerequisite is to have common sense and then a good sense of color.
Next step is the learning process wherein you need to practice on your imagination skills and try to replicate it digitally. So your next prerequisite comes down to software skills i.e learning the design tools and it's usage in a effective way. Good Understanding of Photoshop and illustrator is a must I would say to be a good graphic designer without forgetting the common sense part. And make sure you design for the society's welfare too. The last point is just my request. All the best.

I hope you find this information helpful. I wish you all the best for your future.

Good Luck