Well I am by no means an expert and my talents pale in comparison to many others here but I have been drawing maps for many years and before I started using Photoshop I began by grabbing a pencil and scribbling an interesting shape onto a piece of paper. Nowadays I do almost the same thing but utilize different techniques with Photoshop. It all starts with a cool looking land desigh. Once I have that the shape usually inspires me to add landforms (mountains, forests, deserts, etc). I honestly don't put much thought into whether the location of these landforms are "realistic" beyond having deserts in temperate zones and forests near water. Once I am happy with my placement of these features I work on rivers and lakes, then cities and towns and finally labels.
Like I said, however, I am by no means an expert and there are a very many talented artists on this site that can give you better guidance.