Your files need to have a resolution of 300 dpi at the final size. If you do not have access to professional or adobe programs, then you can also calculate the number of pixels you need. For example, if doing a 4 x 6 size card, you need a file that's created at 4.25 x 6.25 at 300dpi. In pixels, you would need to multiply the dimensions by 300. In this case, for a 4 x 6 size card, you need a file that's 1275 pixels by 1875 pixels. Please keep this in mind when adjusting file sizes for bleed, etc.