Unless your story takes place in a single location, one of the first things you want to do is draw a map. The exception would be if your story takes place on Earth, in which case you’ll be doing research instead of creating your own map.
A map will help you keep track of your characters, plot a journey, give reference for character and monster homes, and give depth and background to your story. You don’t need to be a professional cartographer to draw a map. Chances are high that your readers will never see it. If you do plan on sharing it, you can make it look better later. For now, stick with the rough basics.
Draw an outline of the country, continent, world, whichever suits the needs of your story. My own novel stays within the borders of Dashevona, so I haven’t bothered mapping anything outside that realm. Note the starting point of your story and start detailing out from there. Add mountains, rivers, hills, forests, plains. Make dots for villages, bigger dots for cities. Give names to your towns and natural landmarks. You can be content mapping out only the basics that will be traveled within the story, adding more as needed. Or you can branch out, detailing the entire map. The more you detail, the more material you have for “what ifs”, creating trade routes, and other aspects that can enhance your world.
If you’re stuck where to start, look at premade maps. There are many resources you can use. An atlas, encyclopedia, or Google Maps will show you real world maps. My own map was conceived after seeing a map of Scotland, it’s one big peninsula. You can even trace a map directly, change the coast or borders slightly, and fill it in any way you want. To look at maps other people have created, do a search online for role-playing maps, world-building, and fantasy maps. There are also map-creation tools you can use. Video games like Civilization and SimCity have mapping tools. There is also software you can purchase or download if you want something more professional. An example of these is Campaign Cartographer 3, from ProFantasy Software.
Keep in mind how coasts and mountains will affect weather. You’ll want to research climate and topography. Once you have your map, you can determine how long it will take to travel from point A to point B, have visitors from town C, and trade from D. If you get stuck in the story, perhaps looking at your map will give you ideas. What if this central river flooded? What if this mountain has a cave with a monster? A map is a handy tool indeed.