These days it seems that more time is spent photoshopping images as to taking the time to learn how to compose and shoot.
As panda said - if the image is not there then no amount of photoshop will help.
As this link was hinted to be beginners advice, why not try some basic advice.
A photograph is the product of reflected light from a subject being caught by a camera and recorded on light sensitive film. Now your digital camera electronically records this process or exposure. If your film was not exposed to enough light your image is underexposed (very dark), if too much light was recorded (overexposed) the image would be blown out / white almost. From a digital media point - this is still the case, if you under or over expose your image photoshop cannot repair it. Yes you can tweak the hell out of it but it will never be the same as exposong the image properly.
What affects exposure? Remember we are recording reflected light.
What is the ISO?
ISO sensitivity expresses the speed of photographic negative materials (formerly expressed as ASA). Since digital cameras do not use film but use image sensors instead, the ISO equivalent is usually given. eg 100, 200, 400, 800, 1600 etc
What ISO denotes is how sensitive the image sensor is to the amount of light present. The higher the ISO, the more sensitive the image sensor and therefore the less light required to record the image.
In film terms, faster film say asa 1600 or 3200, would have much more grain (roughness/clarity) than that of a slower film say asa 100 or even 400. Digitally, I believe that this is similar - if others know otherwise I would be interested in how it differs.
What is the Aperature?
The main function of a camera lens is to collect light. The aperture of a lens is the diameter of the lens opening and is usually controlled by an iris.The larger the diameter of the aperture, the more light reaches the film / image sensor. Aperture is expressed as F-stop, e.g. F2.8 or f/2.8. The smaller the F-stop number (or f/value), the larger the lens opening (aperture).
The aperature call also affect the depth of field of an image. A large aperature say f/1.4 will have a very shallow depth of field (blurry background or foregraound depending on your focal point). A large aperature say f/22 will have a very deep depth of field - everything will be in focus.
What is the Shutter Speed?
Effectively, it is how long the shutter remains open. Usually expressed in units of a second. eg 1/60th 1/100th, 1/1000th. Obvousley the longer the shutter remains open the more light that it lets in.
A consequence of shutter speed is blur. Slow shutters require objects to be very still otherwise your image will blur. Fast shutter speeds will capture moving objects as if they were frozen in space.
Taking a correctly exposed image requires and understanding of these parameters and how they interact.
This is basic information that may help. There are so many other adjustments that digial cameras have over their film bretheren. But that is for someone more experienced with digital than I to explain.
I have been working with black and white and trany (slide) film for donkeys (avid amateur) and have been a fan of ansell adams' zone system - which is in-depth stuff about getting the right exposure.
I have only recently ventured into the digital era, and am still getting grips on my new toys. I personally believe that with the advent of digital cameras, people may be less inclined to understand the basic process of how photographs are recorded, and spend more time in post image re-touching than getting the right image in the first place.
I am not a digital cynic, i have seen some fantastic digitally manipulated images. But on the otherhand i have viewed amazing film images - ie Ansell Adams and Peter Lik, Steve Parish etc.
BTW - I have no-idea how to use photoshop, wifey can so that helps

Wormy - hope this has helped.