Time to make history! History for your world that is.
I'll whip out another great Dungeon Master tip for this subject, which is to create 3 major events: One that happened a bit back in the past, but still resonates within the world due to its importance, an event that happened a while back, and current events.
The far past event can range from a great natural disaster that shook the region or world, a great war, the depletion of an important resource, the extinction of a race or species, the death of an important figure that has thrown a realm into civil war, this is something that anyone would know about, or hear in passing.
The middle history is something that happened a month ago, maybe a battle was won or lost, talk of a new faction is overheard, death of an old faction opens an opportunity for those hungry for power to step up. Word of mouth is carrying news of this event fast and can help further set the stage for events to happen.
The current event is something that is best to have more immediate repercussions. A battle begins, a town is set ablaze, the winter sets in, a river dries up, a mysterious murder takes place, the fifth one this week and everyone knows it is not a mere coincidence.
These events can help set a more local understanding of your world if you're deciding to focus in on one area first and expand from there in particular. This might even be a helpful tool to create each country as you line off a map and figure out the concerns of these countries.
You can then think back much further, perhaps even into the concept of creation of this world. Which origin stories are repeated by storytellers around fires? Which history is written and passed along, and which history is forgotten? You most likely delved a bit into the history of your cultures yesterday, but today it's time to really flesh out those timelines and dot them with points of interest and…
GET BUILDING!