For starters, you will want the country within which your characters will be travelling. If the journey spans a number of countries, you should include those as well. The best way I've found to visualise my world is to draw a map. It doesn't matter whether it looks good or bad, all that matters is that you can see where your characters are or where they are headed. Below is an image of the map I drew for my latest work-in-progress, Aundes Aura.
I have called this world Válkia. I believe there are more details to the world, particularly in Arlea, but considering my characters have only just left their home town, I don't think it is absolutely vital that I have those details right now. For the names of my towns and my countries, you may or may not have noticed that I have borrowed heavily from other languages in my quest for interesting names. The name "Tierra" comes from the Spanish word for "earth". The name "Arlea" bears a resemblance to the French "Arles". "Robarre" is what I came up with when thinking about the French pronunciation of "Robert", being "Robear". The capital "Parthon" comes from the French words "by" and "tuna", mashed together. The name of the southern country comes from the phrase "Du thon" meaning "some tuna". Finally, "Mengerikaan" in Arlea borrows from the Indonesion word for "horrible", being "mengerikan".
This is all very random, no? The roots of these names are meaningless to the point of ridiculousness (tuna, anyone?), but they help me name the places nonetheless. Who says that the names have to mean anything?
As you can see, though, each country has a different linguistic influence. Duthonne is very French driven, and Arlea so far has island-like influences, almost like a tribal language. Meira is far more subtle with its names. One town I have not found the right name for is "Anoria", near the mountain range, but for now at least I have something to work with.
Even more so than what I have drawn on my own map, there need to be differences in your landscape to make it feel more realistic and keep it a little interesting. I think my map may be geographically incorrect, but I doubt that matters much. Which is more important? The story? Or whether or not that mountain range can be there due to the placement of tectonic plates on the world? Like I said, the aim here is to keep the adventure interesting. Grassy fields are nice, but if the whole book were set in grassy fields... well, it hurts me inside to imagine that, both as a reader and a writer.
Within or between the countries you have created, you will want some conflict. This outer conflict will be very much related to your World History, and having one will help you with the other. For example, in The Lord of the Rings, there are the creatures of Mordor who act in the favour of Sauron. Pure Evil. But beyond that, there are the men of the east. Then, stacked on top of that, there are the quarrels between the allied countries, between their own countries, and between their own houses or clans.
Maybe in your own novel, there are two equally fit candidates for the Throne--how do we decide who will be King? Someone assassinates one of the canditates, perhaps? But this is not in any way related to the main characters, it is simply backstory to the world, or history. See how thinking of the outer conflict has brought us to create some history, some reality to this world?
Finally, what do you want your villages, castles or landscapes to look like? Will you go early medieval with small, stone houses? In my case, I have chosen an Elizabethan style for my world. To make sure I understood what I was talking about, I did some research to find out how the buildings were made. Elizabethan buildings had a squared wooden frame, between which was woven a "wattle" (sticks and twigs). A white plaster called "daub" was then spread over the "wattle" to create the walls.