There are potentially some really long answers to this question. But the real short answer includes flattening your photoshop file, verify the file is CMYK and request a proof from the press. Or better yet, supply a color matched proof to the press. If you’re working in Illustrator, make sure the photos have been embedded and your fonts are outlined. A final easy option is to save a press resolution (300 dpi) PDF. There is a lot that goes into making a file press-ready but this should get you started in the right direction. When in doubt contact the press’ pre-press department to have them tell you what they need. If this is your first time printing a file, don’t be surprised with a pre-press cost involved