The resolution of a computer/phone screen is 72 DPI. This means that even if an image looks like it is high quality on a screen, it may still look bad when it prints out. 

To check the resolution of an image, you can upload your image here and see what size and quality you can print. 

If your image is too low resolution, there is no way to make it higher resolution. Even if you change the image settings to a higher DPI, the image will still be just as blurry. For example, if your image is 100 DPI and you change the settings to 300 DPI, it will duplicate the pixels 3 times, so the image will look just as blurry (even though it is technically a higher DPI).

Most newer cell phones take high resolution photos, just ensure that when you export your image you export it at full resolution.