When priting photos via professional lab, online or local priting house it's important to understand cropping ratio's and image sizes. All my images are usually cropped to the 2:3 ratio (unless otherwise stated). So if you wish to print other sizes you might have to crop your images due to different aspect ratios.
Especially when you are using photo kiosks you need to select the function that allows you set the crop lines yourself. Don't ever allow the computer to do the cropping as it can crop some important detail from your image.
To help with this I have created a little guide for you - hope you find it useful.
p.s. I will also attach this document to all my USB's and DVD's so that when you purchase digital images you can use this as your "cheat" sheet.