The first thing you might like to address, when you begin to retouch your photos, is white balance. White balance refers to the color of the light and implies that the best light color is white. Some photos, like sunset or candlelight, do not have white light, but usually an impression of white light is desirable.
Some use Photoshop's auto levels to set white balance, but that is not ideal, since auto levels merely sets the brightest pixels to white and the darkest pixels to black without looking at the mid tones. But what if the lightest pixel in your photo is not white? Or what if you do not have pure black in the photo? (Most pictures have black areas, but the lightest pixel is rarely pure white).
The mid tones are the most important and to help set the mid tones correctly one adds a grey card to the image when taking the picture. A grey card is a flat piece of cardboard or plastic of an exact mid tone neutral gray. Ideally one has three cards: a black, a gray and a white. Photoshop's levels adjustment panel has three eye droppers for picking color: one for white, one for gray and one for black. By clicking the gray color picker on the gray card, one can adjust the mid tones to neutral gray. One can only include a gray card in the photo if one intends later to crop the photo.
If one doesn't want a gray card in the image, or if one doesn't have a gray card at hand, one can later use specialized software that scans the image and finds the color of the light and sets it to white. There are problems with such software: what if there are no neutral areas in the picture to deduct the color of the light from? Some applications do not need a neutral in the picture, but most do to get a good result.
If you work with RAW images, you will have discovered that the RAW converters usually come with a slider for color temperature, meaning a control to adjust the picture cool or warm. But what if the color of the light is greenish as when you have taken a picture in fluorescent light? The cool-warm control is good for regular incandescent light, but not for fluorescent.
Color correction sliders are rarely good for correcting white balance, because the color correction will not only neutralize the gray card, but will also tone the image in an undesirable way: usually the blacks become colored or the whites or both.
In short one needs some neutrals in an image to correct white balance. A white wall or a piece of white paper will do well; at best also a gray card for the mid tones.