You can't improve resolution beyond what was captured when the photo was taken. Any processing you can do would just be to alter the look a little, which may improve apparent sharpness or reduce apparent noise but won't actually enhance the detail in the way that you could if you had more resolution from the start.
Assuming that it was a decent modern camera, it should have more than enough resolution in it to begin with even for fairly large enlargements (eg 20 inches long) without it looking terrible.
I would suspect that the camera is not set to record at its highest resolution (check in its settings), or the tool you used to upload to Wal-mart reduced the resolution of the phone as you uploaded it.
Take the original photo file in to a photo store or camera shop and have them print it from that.