This is an exhaustive topic and many experts have written volumes on it. As such, I do not have any aspirations to attempt a comprehensive approach with my limited knowledge of the subject. My purpose is to give a very pithy short course on some simple techniques that I employ, and refer you to some well-documented web sites for more specific detail from far greater talent.
I will divide the subject into some basic topics, and will continue to add new material and illustrations as I progress with my efforts.
Although I use Adobe Photoshop LE, many other similar software packages have very similar features, but you may need to experiment around a bit with their menus to find applications.
The first order of the day of course is image acquisition, which is also a very broad topic, so I will leave you to your own devices, but share with you how I acquire the images with the imaging media that I use, and progress from there by addressing these steps individually below.