If you plan to build an entire planet, there is a lot to do. No geographical feature exists in a vacuum, and the movement of the planet has effects which don't have visible effects for thousands of years.
Assuming your world is a normal planet, it will most likely obey the laws of plate tectonics which geologists study on Earth. I'm no geologist, and my knowledge of plate tectonics is derived almost exclusively from Wikipedia, so don't use this as a source for your Geology paper. Still, a rough understanding is likely enough to get the job done.
There are three types of plate boundaries, each with very different implications.
- Transform: Some plates slide or grind past each other. This causes earthquakes, but no major new geological features.
- Divergent: Some plates move away from each other, leaving a rift in the planet's crust. This allows the magma below the plates to bubble up to the surface, creating chains of volcanoes, and possibly volcanic islands if the volcanoes appear sufficiently close to the surface of the sea.
- Convergent: When two plates push against each other crazy stuff happens. One plate slides beneath the other (the subducting plate). The other plate bends upward, creating mountain ranges. If these mountains are underwater, they may form chains of islands.
Simulating Plate Tectonics
Simulating plate tectonics is hard. You can make things up and draw up a world which looks good to you, or you could try one of the methods below if you want a random and natural feel to your world.
Scrap Paper Method
Take two sheets of paper of the same size. Anchor the first sheet to a solid surface by taping down the corners. This will simulate your globe. If you want, you can choose a Map Projection, and mark the shape of your world with a dark pen or marker outlining the shape of the map projection. This will help you to concieve a realistic globe. If you elect not to choose a map projection, you are defaulting to a Mill Cylindrical Projection (essentially pretending that your map is the outer edge of a cylinder lain flat).
Take the other sheet of paper: mark one side in green, and the other in blue. Tear off the edges of the second sheet so that you don't have weirdly straight tectonic plates. Tear the sheet into pieces of varying sizes, then throw them into the air (or gently shuffle them, whatever). Ideally, some of the pieces will land on the green side, and the rest will land on the blue side. If some pieces manage to land on the edges of paper instead of on either colored side, gently set yourself on fire. The airflow toward your smoldering body will knock the paper over in a reasonably random fashion. If that doesn't sound like fun, either knock them over yourself or toss them into the air again.
It's fine if you don't have an even distribution, but you generally want more blue pieces than green. These pieces are now your tectonic plates.
Spread your plates out on the table with a bit of space. Some should begin on your map sheet. Move the pieces closer together gradually until the map sheet is mostly covered. Observe the points where the plates touch each other. Move things around a bit until you have most or all of the plates on the map, and the map sheet is almost entirely obscured. Take a picture of your paper abomination in case something tragic happens and your world is prematurely destroyed. Note where plates overlap each other, and where there are gaps, and see the information on plate boundaries, above.
Find a third sheet of paper (or use photoshop or something over a picture of your paper pre-map). Draw out the outlines of the continents. Mark mountains where land plates overlap other plates. When sea plates overlap, add island chains. Where there are gaps, add volcanoes, and possibly volcanic islands if the volcanoes form at sea. This gives you the basics of your
Orange Peel Method
Find an orange. Peel it. Press the peel flat on a flat surface. That's your world map. The peel represents land, while the empty space is water.
- Tectonics.js - A browser-based plate tectonics simulator. Not particularly attractive, and it doesn't show elevation very well, but it supports multiple map projection styles, and the different view modes are great for figuring out how to fill in the details.
- SEPUP Plate Motion Simulation - It won't generate a world for you, but it provides a fantastic visual of what happens when plates interact in various ways.
Water covers 71% of the Earth's surface, and it plays a major role as a piece of the Earth's climate, as a means of transportation, and occasionally as a beverage. Life can't exist without water, and even in settings where there isn't a lot of water, the need for water is often a major part of daily life. Unless your world is extremely arid (lots and lots of deserts), water will likely make up over half of your worlds surface, and it will have major effects on how life begins to inhabit your world.
The majority of the watery surface area of your world will be oceans. Oceans can come in many strange shapes and sizes, so it's basically impossible to get this part wrong. Look at however the water came out when you generated your world using tectonic plates, and call that good. Any bodies of water directly connected to the general ocean area will be salt water, but it's possible that land-locked bodies of water might be large fresh-water bodies like the Great Lakes.
Rivers and Lakes
This step is where I see most people go wrong. Rivers are conceptually very simple, so long as your world follows the real-world laws of physics. First, gravity affects water, so water tends to flow to the lowest place possible. Second, water is a liquid, so it flows to fill it's container.
Where do rivers come from?
Mountains, generally. It's important to note that when you look at a river, it isn't that size for the entirety of the river's run. Rivers start from very small trickles of water, some of which come from natural springs, and some of which come from melting snow. These tributaries flow downhill, gradually aggregating into larger rivers until they eventually reach the ocean.
Where do rivers go?
I'm going to quickly note that rivers do not split. It's not a thing. There might be an island in the middle, or a particularly large rock, but rivers absolutely do not split into divergent paths and continue on their merry way. Rivers flow downhill, always downhill, and only downhill. If there are two choices, water chooses whichever is the most downhill.
As mentioned previously, small flows of water join with other tributaries as they flow downhill, and gradually form into a larger river before flowing into the ocean. If you look at a map of a river and its tributaries, it will look startlingly like a two-dimensional tree. Imagine the tips of the branches as water sources, and the trunk as the largest section of the river.
The most notable exception to the "rivers don't split" rule is a River Delta. River Deltas a large, flat area which are extremely close to sea level. Rivers split between clusters of islands, and move very slowly into the ocean because there is little or no change in elevation to direct the flow of the water. Continuing my tree simile (I said "like", so it's a simile, not a metaphor), the exposed roots of our two-dimensional tree are a bit like a delta.
There are extremely few other exceptions to this rule, and several of them are man-made.
How do lakes form?
With great difficulty! Compared to rivers, lakes are exceptionally rare. For a lake to form, a water source (frequently a river) needs to flow into a basin of some sort. The water inflow must at least equal the outflow of the water, keeping in mind evaporation, water seeping into the ground, and the likelihood of a river which drains the lake. Most lakes will have one or two inlets which feed the lake, but will typically only have one outlet for the same reason that rivers don't split (gravity).
There are some lakes which lack permanent inlets and outlets, like Rattlesnake Lake in Washington. These lakes are filled by rain or melting snow, then gradually lose water during dry periods, or may have outlets which form when the water level grows high enough to overflow the edges of the lake. Lakes of this sort are common in places like Minnesota.
It's difficult to provide examples without visual aids, so bear with me here.
Example 1 - Shadow of Olympus
Go to google maps, search for "Mount Olympus Greece", and draw a circle.
Example 2 - Space Grease
The real-world solar system already has plenty of planetary geography that we can steal.
Example 3 - Heroes of Tonesvale
We don't need to design a whole planet for a single-city setting, but a bit of local geography is a nice touch. For the city to be reasonably interesting it needs to be of reasonably large size. Large cities tend to occur at major shipping hubs like ports, and water is always a cool geographical feature. So let's say that Tonesvale is based at the mouth of a river on a large natural harbor. Flat land is super boring, so let's add some mountains nearby, and a couple of lakes. Try not to re-create Seattle too closely; it already has superheroes.