The printer is probably the best person to ask, and will be able to help you supply the right file, and at the right size, and should be able to adjust the file contrast, values etc for the machine he/she is running on. Ask for a proof before letting them print it though - just to check! They will want a .tiff file, CMYK and the size 30 - 40 megs is about right for an A4 size page.