Editing photographs

The truth in every camera (reflex or digital) is that it doesn't show the colours how they are. When you shoot, the colours aren't like in real life. This is because our camera can't show them with the same contrast, saturation, brighteness...
Because of that, you can increase this points with a photograph editor. The most popular ones are:
Download (free and paying)
PhotoShop (not free), PhotoScape (free), GIMP (free), (it is more focused on drawing), etc.
Online (free, or free basic tools)
Pixlr Editor, PicMonkey, SumoPaint, FotoFlexer...

In my opinion, the less edition you make, the best your pictures look like. I mean, photos with too many filters look harder on our eyes. A simple edition make your photos look better, so the attention is focus on it and not in the heavy edition.
If you want to catch the attention in the edition and not in the photo (because it is too simple, not artistic), it's better to know the basic edition tools. You can search for tutorials on DeviantART or in google.