Do you have a background?
If you have a background, that needs to be removed because its not part of the print, there are many ways to get rid of them in Photoshop. Our favorite is the magic eraser tool. This is located with the regular erase tool. You might have to click and hold to see this tool pop up. This allows you to click to the background color and it will remove any pixels of those colors. You can use the tolerance setting at the top to choose how particular it is, but the default setting of 32 works well enough in most cases. A lower number would be more picky about the shades, while a higher number would be more lenient. The important setting to pay attention to with this tool is the contagious checkbox up top. If it's not checked it will select pixels of that near color anywhere within the art. If it is checked it would only select that color, and its selection that directly touches it.
In the image below you see that I have two layers. One is my artwork that will print, and the other is a bright green background. This color you use for your background servers two purposes. It can mimic the tee color to gauge the final print, and it can provide an obvious background that bright, so we can tell exactly what's print and make sure we didn't miss any random piece of art when we deleted the original background.