Diversify

If you run a world that suffers from uniformity, where all the countries seem the same, and where one town the party finds is a cookie-cutter semblance of the town before, and the town before that ... then you must invent diversity.  For your first go-round, I urge you to go slow.  Think of the marks you make on your world being like the symbols a tattoo artist adds to skin.  Whatever you do, you want it to be permanent.  You want not to regret it later.

What is diversity?  It is the pattern a society develops because of its land, resources, climate, historical happenstance and moments of genius that have popped up year-by-year.  A region becomes militaristic, idealistic or factionalized because of its relationships to other regions, the certainty of peace and its relevant wealth.  Some countries are destined to be slaves; others, to be masters.  Without Earth's history to sort this out for you, you will have to look at your world map and settle this yourself.  Don't fall into the mistake of thinking every region wants the same things; or values one universal treasure above all things.  Not every country loves money like Americans do; or religion like the Italians do; or solitude like Russians do.  Find a distinguishing, unique personality for every region ~ but like I say, do not rush into it.  Give yourself a great deal of time and thought before making a decision.

There is an old trope called "planet of hats": a condition where all the inhabitants share a single defining characteristic.  I don't encourage this as a solution, but I do urge the reader to use it as a jumping off point.  As TV Tropes explains,

"To some degree, this is unavoidable; you have only so much screen time or page space to develop and explore a culture."

As a DM, you have scores of cultures to create, so to some degree you're in this same boat.  But if you give the matter thought, you can see how the trope can be a short term solution, leading to greater and greater specifics ... in the way that the Klingon culture evolved over different series, as more and more thought was added.

Initially, you can designate a generalized wardrobe for five or six different cultures; perhaps, in some cases, for specific character races.  This wardrobe can be defined by the types of clothing worn, but also by its color.  Perhaps in the country of note, a particular seashell enables the cheap creation of a violet-bluish dye.  Or perhaps orange, or brown, or white.  Coupled with a clothing style, a player character moving down the street can tell the locals from the foreigners ... and we may suppose the foreigners are proud to wear the colors and clothes of their home country.  Just as they've all been trained to use a select kind of weapon, or carry specific charms or types of carry-all.  Perhaps they wear a kind of shoe, or like a particular beverage or food, one that reminds them of home.  Then, if the player stumbles across a bar deliberately made for members of another region, every one is suddenly wearing violet robes with headbands, and speaking in deep accents or another language.  If the players are looking for information about that region before going there, they should look for this kind of establishment.

Don't rush into making a bunch of details for every place in your world.  Make a few; and then as you run these groups in a campaign, let your imagination expand on what's already there.  Within a few dozen runnings, you should have three or four somewhat fleshed-out cultures ... and your players will begin to recognize them, and perhaps ask specific questions about what those people do, or like, or believe.

Thereafter, it can become much easier.