For the best possible results, start by downloading an Adobe InDesign file for your chosen product size below. Use the InDesign file to layout your design, and export as a high resolution PDF.
Many products also have their own individual templates that you can download. Just go to an individual product, scroll down and look for the "downloads" tab.
Remember to use CMYK colours, and high resolution images (around 300dpi) in your designs so they come out looking great.