Overview
There are many overlapping and alternative photo correcting procedures, some are even automatic. Examine the image and suggest changes needed.
See the links for some details of relevant steps. More will be added .
Prevention is better than cure.  Review how to use your camera!
A suggested work flow might be to consider  these procedures and use what is appropriate
Work on a copy of the image. Duplicate the background image to easily refer back to the original state.   
As you work:-
       Name layers appropriately to reflect your intentions
       Mask areas to exclude them from adverse changes
       Make global corrections and then local corrections
Consider file size, file format,  and saving the image
 
Straighten
Crop
Levels
Curves
Shadows/Highlights
ColourCast correction
Sharpen

Global Changes

Aim at adjusting  Brightness, Contrast , Colour Balance  (hue),  Saturation   (make colours more or less intense)
Straighten an Image
Six Named Adjustment Layers 
       All are editable
       Nothing alters the original image
       Use the best adjustment layer for the job. Not necessarily the lot!
 Mask areas as necessary     (See where mask is painted with “ \ ” key)
Use layers (to preserve original pixels) 

Using Layer Modes
Duplicate the image layer and use Multiply to darken a light image
Duplicate the image layer and use Screen to lighten a dark image
Duplicate the image layer and try Overlay on muddy images
Consider using “Luminosity” layer mode for adjustment layers to only change brightness and contrast value – not saturation
Use layer opacity o vary the intensity of the changes

"Levels"  for brightest and darkest
"Curves"  for mid tone contrast”
“Curves"  for Colour balance”
“Selective Colour" for changing colour balance
"Hue /Saturation"  for Saturation
Shadows and Highlights" - Convert the image to a Smart Object .  Shadows and Highlights then becomes available as an editable adjustment layer

Selections can confine changes
Dodge, Burn & Sponge tools
Clone, Healing,Spot Healing and Patch tools
Vanishing Point Filter

Cosmetics

Red Eye
White teeth
Smooth skin

Building an Image

Replace the Sky
Collages

Some Printing notes