The most importantest possible thing to know of all things that you might ever know about maps: They Are Inaccurate. Then you get confused about your map, go back and re-read The Most Importantest Thing.
The second most important thing you'll need to know is: Why Does The Map Exist? Is it to sell bits of land to unsuspecting folks in a different country? Is it to keep out undesirable people? Is it for sheer bragging rights? Is it to give a skeleton on which to hang a set of stories? Is it to try to keep stories consistent with each other? Is it to help a ship's captain to find his way home? All of these possibilities result in very different maps for the exact same physical things! Mapping is about abstraction and which things get abstracted away and which get marked down as important goes directly back to why the map exists (and who made it, to some extent because there are some real @#)*&^ jokers out there).

From what you said, I'll guess that you're looking at a medieval-ish "points of light" setting where most of the landscape is relatively unknown and there aren't a whole lot of high-precision mapping crews out there generating maps using their GIS software based on satellite photos. You should be able to mark down some places that are important, some connections between them (road and water), and some blockages that prevent people walking directly from one to another (high ground and low ground full of water are the usual barriers).

The simplest heuristics are:

Mountains (especially impassable ones) tend to come in chains. Look at most maps about the size that you care about. You'll see a handful of mountain chains on the order of 1000 miles long. Put those chains where you don't want people to just wander back and forth. Now go look at a map of the Carpathian Mountains in eastern Europe and try to figure out exactly why Mordor is flipped East-to-West. I'll wait. You're back? Good. There are reasons why mountains are exactly where they are, but Priest Bubba drawing pretty maps seven generations removed from the survey crew who mostly drew their maps based on rumors of their area of interest probably isn't going to get it quite right.

Water gets to the tops of the mountains because that's where snow accumulates. The physical world is like a teakettle with the sun boiling the oceans and the steam condensing out in high places. Then that water runs downhill until it gets to the ocean or evaporates in the desert. Liquid water flows downhill (solid water flows downhill more slowly). That's the most important rule of geophysics (water dissolves stuff is the second most important rule). So sketch the lines going from high to low and call those rivers. Rivers usually go away from mountains, but some like to follow along the bottom of the mountain range (like the Po) and some just want to see the sights on the way to the ocean (aka the Danube). Go to your search engine and ask it for things like "map of rivers of Europe" and win a fun prize!

As far as where people will settle, consider that they'll want water and food. The highest fordable point on a river is a good spot (Rome wasn't built in a day, but it was built in that spot for a particular reason). An important rule of modern sanitation is that people won't drink their own waste, but will happily drink that of the people upstream. So all water intakes are upriver (ideally from a different river altogether) and all sewer outlets are downriver.