Nauman,

Maybe these screen shots will help.

First set the width and height, leaving the resolution blank.

http://myweb.cableone.net/lenajones/crop1.jpg

Next drag the crop tool from one corner diagonally acrosss the image.

http://myweb.cableone.net/lenajones/crop2.jpg

Your camera format probably will not be equal to your desired image size so left click in the center of the image and drag the crop area to the desired position.

http://myweb.cableone.net/lenajones/crop3.jpg

Crop by clicking the check mark. If you desire to resample or change the resolution afterward then use the image > resize > image size box to do so. Resample only if necessary and if your camera has a native resolution of 240 ppi at the desired photo size then resampling is unnecessary.
If the resize box shows 72 ppi then uncheck the resample box and change the size to 6 x 4 to show the ppi that will print rather than the monitor ppi which has no bearing on anything except what you see on the screen. At 72 ppi it should show a size of 20 inches by over 13 inches, means nothing, uncheck resample and change one size box to the desired size to see the ppi at the size you wish to print. Hope this makes sense to you.

http://myweb.cableone.net/lenajones/crop4.jpg

Saving as a JPG is best done at the highest setting of 12 for printing. It is best to save as uncompressed tiff or psd if possible. I don't know which file formats Walmart accepts but you might ask if they will print from tif files.

http://myweb.cableone.net/lenajones/crop5.jpg

I hope this helps. Don't hesitate to ask questions. CaseyJ.