Presuming your photos are of good quality to start with (no blur, no heat haze, no overexposure) it is just a matter of learning how to use PS
. If you know anyone who already gets photos accepted on A.net, I would suggest you ask them to show you how they do things....learning how to fix photos from manuals is very hard. There is a lot of art involved, not just science.
Most important thing is to know what you want to achieve with a picture before you even get started. Also, be realistic. Don't try to fix photos that are too tricky to get right (like shots on cloudy days). Concentrate on getting some easy ones first....like well lit, sunny days.
As far as PS
....learn how to use layers. I wasted a lot of time on fixing photos till I learned how to take advantage of layers. Another very useful tool is the Viveza plug-in. It does wonders for bringing out detail in shadowy areas (helps avoid "dark" rejections)
Contact me if you have more specific questions.
[Edited 2012-11-13 10:00:59]
I'd rather be flying.