Generally, files should be CMYK. Grayscale if printing as black and white. PMS if printing with spot colors. You may have a file that has all 3 color modes printing, If you were printing 5/1 for example. This might be 4 color process plus a 5th color PMS on the front and grayscale or black on the back.
By default, many design programs such as Adobe Photoshop are set to RGB. Before you begin working on your files, it's best to change your color mode to CMYK. (By the way, all our templates have this preset for you.) Photoshop example:
Adobe Illustrator is also defaulted to RGB, you can change that to CMYK. Illustrator Example:
Further color management can be altered in Illustrator for your monitor and separately for CMYK printing. US Sheetfed Coated is acceptable for most of our paper stocks.