By now, the world has oceans and mountains and some climate and weather patterns. I look at rivers next because I want to know where the water is going to be, because generally water means life and the more water there is, the more life there is, and how much water there is controls how much life there is (glibly, but then all of this is glib, and temperature and sunlight have a big influence too – provided there’s water). Rivers also have another purpose though, because rivers can flow through places where there is little rainfall and yet be the basis of civilisation (ancient Egypt revolved around the Nile, and other early civilisations of the middle east revolved around the Tigris and Euphrates). Rivers are also a means of navigation, a relatively easy way to get from one place to another through what might otherwise be hostile territory, be it dense forest or desert.

This may sound stupidly, obvious, but I’ve see maps that fail to obey all of the following simple rules: Rivers flow from high places to low ones. They tend to get bigger and slower as they move from mountains to the sea. In places where the incline is steep, rivers flow fast and tend to be in steep-sided valleys, gorges or even canyons. Rapids and waterfalls are more likely to appear in such places (but some of the world’s largest waterfalls don’t). Rivers merge together rather than diverge, the exception being deltas. In places where the gradient is steep, the course of a river is determined by the topography; in places where the gradient is shallow, rivers flow slowly and have wide gentle valleys and/or floodplains and are likely to meander. Map your rivers with these in mind, but also considering that rivers in warm places will likely be the arteries of the early civilisations of your world.

As usual, when it comes to basic geography, the simple way of finding out how stuff works it to look it up on wikipedia

Wikipedia on rivers. More basic river formation articles here and here. Also Deltas, Canyons and Valleys

Next maybe something on glaciers and fjords, because I like them. And then ecologies and the tricky stuff starts…