Firstly if your unsure PLEASE ASK and we will try to help.
Direct to Garment printers are larger industrial purpose made versions of an Inkjet printer and will produce high quality prints. However they can only output the quality that is sent to it.
If your file is blurry and low quality that is how it will be printed on the t-shirt. The site software will display warnings when the file quality is low. But if your file is high resolution but the image is pixelated the system will not be able to detect the issue.
Before uploading please zoom in to check you are happy with the image quality.