For the sake of this discussion let's assume you're printing on a Canon inkjet printer at 300 dpi. You are aiming to make a 21" X 14" print to frame and sell.
A 21" x 14" photograph will be printed using 6300 X 4200 ink dots.3
But your digital image has only 5760 X 3840 pixels!
For best quality you need to resize and resample the image so that it will have 5700 X 3900 pixels. Then, when you print, 1 pixel ➝ 1 ink dot.
The pictures here show the process for resampling using Photoshop Elements. Most applications will be similar: You will need to specify the print size and the print dot resolution (300 dpi for Canon or HP, 360 dpi for Epson)
Resampling is a complicated business. Fortunately most photo applications have built in routines to do it, and to do it better than a print driver alone. If the image is to be upsampled, as in this example, if you have a choice you should choose to resample using the "Bicubic smoother" resampling algorithm.