The only way to print photos without cropping is to keep the photos within the correct aspect ratio. So if your camera took the photos in an aspect ratio of 3:2, then you would need to print them in the same ratio (so either 6"x4" or 12"x8"/ 10x15 cm or 20x30 cm). If your camera took them in 4:3, then the correct print size would be 8"x6"/15x20 cm. If you try to print in a different ratio, then the photos will be cropped.
Please note: when you upload a photo, the editor will automatically choose the right measurements for the print and it will give a warning that the photo has been cropped and/or if the resolution is too low. You first have to approve this before you can go on to editing and ordering your prints.
To edit the cropping, click on the thumbnail of the photo. The photo will enlarge. Move the photo around by holding the mouse and dragging the photo to edit the cropping. Click on "Save" to save the changes.