If I'm designing for a black shirt how could i send my design should it be black and im going to instruct them that it should be white or should i make the background black and the design white
The T-Shirt printers should be able to provide you with the information.
However, as a general rule for single colour white printing on a dark background you should provide them with the design in BLACK on a white background (reversed), but accompany it with clear instructions and a visual of how you want the shirts to look when the design is printed in white ink.
If you've created the design yourself, you could make the image transparent and coloring your design with the color you've thought of (not dark colors, people will have a hard time reading/looking at it leading to frustration)
But you still need to give instructions so the t-shirt printers would know what the output you envisioned.
If you intend to print white onto black shirts, create the design as such. If you're using photoshop, export with a transparent background.