First major attempt at world building.

[Mendicant]

My first step in world-building is to draw a map. Outline the shape of the landmasses that the campaign takes place on, then put in mountain ranges, followed by rivers and lakes, forests and deserts, and finally important settlements and adventure locations. Having some sense of where things are is especially important if you're running a campaign with a significant exploration element to it.

When you're drawing your continents, it can be helpful to think about how you want the PC's to travel when they explore. At lower levels, geography is important: sailing will be the fastest, most convenient method unless the landmasses interfere with sea travel,--think about how Africa made the silk road necessary--so if you want dangerous mountain crossings and desert caravans, lay things out such that those crossings are necessary or efficient.

You don't need to plan all of this geography out to the n'th detail up front, but brainstorm a few set pieces that you'd like to include and make sure there's a spot on the map where they make sense.

Once you've got the skeleton of a world, the next step is to populate it. To start, maybe sketch out 3-5 groups, and go into more detail on 1 or 2 of them. When you're making cultures and factions, concentrate first on the basic pieces the PC's will interact with: How does this culture fight? How does it practice magic? Who does it pray to? Who's in charge? What kinds of gear does it sell? What major threats does it face? Answering those questions can help you think of the classes its notable NPC's belong to and some possible adventure hooks. Flesh them out by whipping up a handful of NPC's and pick out a few cultural highlights that make different peoples feel distinct. Real-world cultures are deep, complex things with centuries of history, but, luckily for you, the PC's are basically heavily armed tourists, not anthropologists. A couple of details--dress, musical instruments, food, titles, arts and handicrafts, deep-seated historical grievances--can help avoid the setting feeling like a samey pseudo-medieval sludge, and as you play in the setting, other details and customs will occur to you.

You don't need to drop these details in one big infodump, and you shouldn't. Weave a detail or two into other interactions or scene-setting.
"The Boyar greats you warmly. 'Come! Sit and have some picked duck beaks with me while we discuss this goblin issue.'"
"The caravan that has hired you is carrying a rich cargo of Vostashi brocades and persimmon brandy, to trade for the Mountain Clan's jade and furs."

Lastly, starting from the ground floor means you can invite your friends to help build the world. While you're concentrating on the places they'll be adventuring in from session to session, the age of exploration theme makes it really easy for them to provide some details and inspiration for the "old world" via character backstories. It saves you work, opens the door to ideas you might not have thought of, and invests players in the world.


Edit:
Also, browsing art can really help you get a feel for people and places. Pinterest is great for this--there are a lot of D&D/Pathfinder pinboards, and the pin function lets you easily grab the images and organize them in one place. I also use a flickr account to host easy to navigate collections of art that players can browse whenever. That way, they can get a feel for NPC's or locations if they want to without a bunch of exposition during sessions.