You can learn to design printables with free options like Canva, or Paint. These programs are fine for basic designs, but can be very limiting. If you would like completely custom designs, programs like Adobe Photoshop and Illustrator are excellent.
However, if you want to create something custom to fit your needs, you can learn to create your own! You'll need to choose the right software and learning program, and you'll be well on your way to making amazing and unique printables.