Whatever you need done-- the outfit doing them will like you a lot if your artwork is in a file they can manipulate; such as CorelDraw (.cdr), Adobe Illustrator (.ai), Windows Metafile (.wmf), Encapsulated PostScript (.eps), Scalable Vector Graphics (.svg), to name a few-- so that they can isolate "layers" to do what's called "color seperations" or "half-tones". Vectoring is the standard for creating and moving the image and informing a printer or silk-screen burner where bits of the image/text start and stop and what color value those bits are supposed to be.
A Jpeg for instance, won't work since it's basically a picture of the file image; not the file itself.