Before you begin editing photos, it is a good idea to calibrate your monitor if you can. This will mean your prints more closely match what you see on screen, preventing time-consuming and wasteful trial-and-error. The most effective way to do this is using a calibration kit, but if this is too big an investment to justify for just an occasional photo print, here are some quick fixes to try.
First, it is very common for prints to look too dark, simply because your monitor reveals more detail in shadow areas than can be printed. Modern screens all support a much greater range of brightness than prints do, with a contrast ratio typically at least 1,000:1 and often much more. In comparison, the best glossy photo paper might achieve a 200:1 contrast ratio between the darkest black and the brightest white, while a matte paper might be only 100:1. For a more accurate on-screen impression of what you'll get in a print, simply turn your monitor brightness down.
Second, modern LCD displays tend to be too blue, particularly when new (the colours often become warmer – more yellow – as the display ages). To remedy this in Windows, go to Settings > Display > Calibrate display color. On a Mac, go to the Help menu and search for Display Calibrator Assistant. Displays typically have a default white point (colour temperature) setting of D65 or 6500K – try changing this to D50 or 5000K. Or, more simply, set the white point so that white on screen matches the white of the paper on which you are printing, under the lighting conditions in which it will be viewed – and take care that the brightness of the screen does not fool your eye.
Finally, check your colour settings. In DPP, go to Tools > Preferences and click Color Management. Don't worry about the first option, Work color space – professional photographers sometimes shoot in Adobe RGB, a wider colour space, but if your shots are sRGB then there's no practical benefit in changing them. In the Printing profile pop-up, select the printer you will be using. DPP applies colour management using ICC profiles for printers that support these, which contain details of the exact colour capabilities of devices and enable better colour fidelity.
Next to this, for Rendering intent select Relative Colorimetric. This setting means that if an image contains any colours that fall outside the printer's gamut or colour range, then they will be converted to the nearest printable colours but the rest of the image will remain unchanged. If you select Perceptual, then all colours in the image will be adjusted so as to preserve the relationship between them, and you often find that colour images become desaturated and appear dull. Colours will generally change less if you use Relative Colorimetric.