Step one is don't do anything you would normally do! Photos turn out much better with a little enhancement, but doing anything other than setting the white balance properly is like to get your photo thrown out. Do not adjust the color, the saturation, the curves, nothing. Keep your hands away from the toolbox!

click to enlarge
What you do want to do is crop your photo to the right dimensions. A passport photo must be 2 inches by 2 inches, so set your crop tool to 2x2 and zoom in on your head and shoulders. You want your head (from top of hair to chin) to take up from 1 inch to 1 3/8 inch of the photos height. You want your eyes to be 1 1/8 to 1 3/8 inches from the bottom of the photo. There can be some resizing here, but be careful to not degrade your photo quality or it will get tossed. The best bet is to resize and see if it works. If not, don't resize again from there, undo all the way back to the original photo and resize again. Repeat this until you have it right. Keep in mind your neck and shoulders are supposed to appear in the picture.
(Image from official US State Department Passport Website)