Expanding a photo to 16x20, how can i get a higher resolution?

Hello. I know - I'm a novice and it's probably a basic question. I'm trying to take a digital picture and print it into 16x20 but the image becomes super grainy. How can I improve the resolution so that I can print it and it's not so grainy?

By @rachelle_5233

What software are you using? Please be specific.

What are the pixel dimensions of the image?

If you want to print the image at 16 x 20 inches at 300 ppi, you need ideally 6000 x 4800 pixels.

Physical dimensions in inches multiplied by ppi value = required pixel dimensions.

@rachelle_5233   You need to post specific info, e.g. 16 by 20 what inches or centimeters?