In Illustrator, you can design patterns, complex graphics and work with type. You can learn to design printables fast with the Basic Printable Design Course from Printable Crush. Illustrator is a highly in-depth program, so taking a course is a quick way to learn the basics.
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.