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.
The biggest question is whether you should get an inkjet or a laser printer. It depends on your needs. If you plan on printing a lot, the toner in laser printers lasts a long time, but the replacement ink can be pricey. The color on laser printers is much more vibrant than an inkjet. However, inkjet ink is relatively inexpensive but plan on replacing it often. Inkjets also print photos better, so if you use photography in your printables, that is something to consider.