Eric gave some great advice. I would just like to add: You should find out from your printer what file format (jpeg, tiff, ??), color space (sRGB, AdobeRGB, ??), and image resolution they prefer or recommend. For instance, while 300ppi is often used as a "standard" for high quality prints some printers work better at a different resolution, and providing the file as recommended by the printer will usually result in the best quality image.