If you're interested in writing fiction in certain genres, or play tabletop role-playing games, there will come a time in your life where you're going to have to draw a map. Jonathan Roberts, who illustrated The Lands of Ice and Fire (A Game of Thrones): Maps from King's Landing to Across the Narrow Sea, has written a great beginning guide to doing just that.
The first thing to remember is that towns aren't just built, they are planned in areas that give them certain advantages. So you can't start with the layout, you have to start with understanding why the town was built there.
"Towns adapt to their surroundings," Roberts writes, explaining the process. "The first thing to do is to draw the terrain the town sits on. In this case I’ve picked a peninsula with a larger outcropping at the end. The coast is rocky and broken apart from a low bay on the NE."
Once that's done, you begin to see how the town would be laid out.
"Once you’ve placed the terrain, use that to inform the locations of the main buildings," he continued. "Here the castle goes on the highest promontory, with a commanding view of the sea and the land around. The cliffs on the promontory provide natural defenses. Any land based threat must come down the peninsula, and the town will want to defend the harbor, so it’s natural for there to be a wall across the end of the peninsula."
There is much more to the guide, and he's written other tutorials that go into more detail, but this is a great step-by-step look at how to create better maps for your games or fiction. Get to work!