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.
Another handy tool for printable assembly is a score board. You'll use it to make creases in paper for easy and precise folding. Don't feel the need to buy all these items at once. It takes time to acquire all the tools for printables, but in the end, you'll find that the investment will be small for a fun and creative hobby or business in printable design.