What does your fictional world need?
I never make a very convincing world unless I consider these steps. I found a diagram on Pinterest early on in my writing career and have since been using variations of that! Here is essentially what they tell you is needed for a successful world. Let me know if there's anything else that you use or think about!
A functional society – determine what kind of society it is and how many people are in the population. Determine how crime is dealt with and how often it occurs. Determine who matters and how doesn’t, and how they clash.
An understandable economy – determine what is used as currency and how it is achieved. Determine what wealth looks like and how it compares to those that don’t have it. Determine what they import versus what they export.
Religion – determine what they believe in (gods/goddesses vs one god; myths and stories) and how they worship. Determine any rites of passage they must go through to be accepted. Determine the role religion plays in society and how it clashes between beliefs.
A complex government – determine how your world will be run. Determine how many people are in charge of decisions, what happens when they clash, and who is in charge of who. Determine the feelings subjects have about the government, what caused these feelings, and how they are dealt with.
Thoughtout Physical geography – determine features of the world. Determinate what the climate is, what natural physical features are present (mountains, caves, islands, volcanos, etc.) and how they impact the world. Determine natural resources and terrain. Determine how people live in the world (towns/cities, nomads, villages, etc.).
Reasonable transportation – determine how the society gets around. Determine if they travel via cars, trains, boats, planes, creatures, ferries, teleportation, by foot, etc. and determine how this impacts their daily life. Determine if all modes of transportation are available for everyone or only certain subjects. Determine which of these options is reasonable with the land features you created (probably need a boat/ferry/submarine if your world is mostly water). Determine how they relocate to other areas (by boat, bus, ferry, etc.) and any regulations with that.
If you don't already use it, Pinterest is a great resource for getting free writing tips!