In this aspect, I personally recommend Picasa from Google for beginners. There's Adobe Photoshop and Adobe Lightroom for heavier and more professional edits, sure, but most of the time Picasa can do the editing that you need.
In this tutorial, I'll share an editing tip that I commonly use: Fill Light and Shadows.
You'll be able to turn this picture (you can click the photos for higher-res versions):
(That's me, in Batulao, Batangas.)
Into this:
Notice how the colors became more vivid? The contrast between the rolling greens and the sky became more apparent.
So how do you achieve this? Simple: Open up your image with Picasa (the main Picasa app, not the photo viewer; if the viewer shows up click Edit in Picasa). Click the Tuning tab on the left side, and drag the Fill Light slider to just about the middle. Then, drag the Shadows slider to about 1/4 to 1/5, or to your desired value. What this basically does is heighten the amount of light that the picture produces by making the colors brighter, and then darkening the shadows of the picture to greatly improve the picture's contrast. The end result would be much more vivid colors and sharper image.
If you want, you can also increase the saturation (Effects tab -> Saturation) to further improve the vividness of the picture, but don't overdo it: oversaturation can result to less-realistic looking pictures.
Some reminders:
- You don't always have to follow the values of the Fill Light and Shadows in this tutorial, as with any post-processing procedure, the adjustments values are always dependent on the kind and quality of picture. Always adjust to your liking.
- Don't overdo. A more subtle change is always more effective.
- The 'safe zone' for the Shadows would not exceed half of the slider, the Fill Light 3/4 of the slider.
So there you have it! Enjoy editing away.