It's a common joke throughout the Media that anything can be 'fixed in post.' The first basic point to get right is that you need a good image to start with; don't assume that editing fundamentally change the quality of your image - more often, it is a matter of enhancing what is already there. Composition, light, exposure, colour - you still need to think of all these things when you are actually taking the picture. To put it another way, you need to start with an image which is worth editing.