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.
If you'd like to create hand-lettered printables, consider taking a custom lettering class so you can edit your designs in Illustrator. Hand lettering is an excellent way to make a printable truly custom.