Trying to run a DND campaign for the first time but I don’t know anything about map making, geography, or geology. I want the physical features of the land to mostly make sense from a geological perspective and then conform the borders of my city-states and empire to their natural geographic constraints. How do I even begin with this?
You could watch a couple of these.
Or you could just decide where plate boundaries are and what their outlines are (for a continental shelf), maybe add some volcanic hotspots that add mountains or islands. Water flows from high elevations to low elevations so rivers and lakes are mostly a direct result of terrain. Don’t worry too much about biomes being realistic, a lot of this is mineral-dependent. There’s a “tree line” above a certain latitude and above a certain altitude where you start to only see grass and shrubs. Throw in a Foehn effect (like how the leeward side of the Rockies and the Andes is super dry, while the windward side is a rainforest) and you’ll be fine.
Maybe consider what “interesting” landscapes you would like to see and work everything else around it.
You can always have clearings and/or plateaus in mountain ranges, you can always have rivers that widen into lakes, or do clever stuff with rias and deltas and estuaries and coastal islands (or fjords if you have mountains along the coast).
Place your political borders however you like. If your players don’t approve, have them stare at the borders of Jalisco and Madhya Pradesh, and ask them where is their god now. Then make their god have a manic episode and give their characters random stat buffs and penalties.