Generally speaking, you will need a JPG or PNG file with a 300 DPI resolution. However, a lower resolution file might also be sufficient depending on the product.
If the product requires large-sized files (like leggings, blankets, or tapestries), then the required resolution will be 120-150 DPI.
The specific file requirements can differ for each product. You’ll see the file requirements for each print area as you open the product in the Mockup Generator (once you click Start designing):