Getting it printed
The PNG exports at 3600 × 4800 px, which is 12 × 16 inches at 300 DPI, with a transparent background so the shirt colour shows through. That is what Printful and Printify both want for a front DTG print — upload it, position it, order one.
Choose the ink to contrast with the garment: light ink on a dark shirt, dark ink on a light one. If you plan to sell rather than just wear it, export one light version and one dark version and set them up as separate colourways on the same product.
Keep the line weight at 4 px or above. Below that you're asking a garment printer to hold a line about a hundredth of an inch wide, and it will either fade or break up in the wash.