The legal protections you can get on a 3D printing vary based on exactly what you’re printing.
If you design and print a sculpture, something that has entirely artistic or decorative purpose, you’re eligible for copyright protection. Luckily, the protection begins once you make the item. If you’d like to monetize it down the road, you can register the copyright, which gives you additional benefits and protections.
If you design a tool, machine, or something that is useful, you may be eligible to patent your new product or device. In order to obtain the patent, you’ll have to apply for the patent within a year of the first public use, or offer of sale of the product.
In certain cases you may be eligible for a design patent on something otherwise sculptural. Design patents cover new designs – and are more limited in scope than utility patents.