I'm writing a story, and it's based in a fantasy world (but it never actually says the name of the planet or anything). The town names are made up, though, and so is basically everything else. It's based in a time similar to the middle ages. What i'm trying to do is create the world so that it sounds real. How do i make a completely made up place take on real-life characteristics? and how do I make the characters react to this environment?
You are asking some very broad and vague questions. I recommend reading books similar to what you're trying to write to see how other writers have approached it. There really is no way to teach someone how to write other than to give him a good book.
Every story has an environment that if it isn't a real environment, has to be made up. You can base your world on a place you have experience with. Think Harry Potter... The story was based in England, but not really. When you make a place up you need to get it right in your head. The easiest way to do that is to draw a map, add place names, road names (if there are roads), landmarks, etc. If you get it right on the map, you can get it right in your head, and then you will be able to describe it on paper. You have to allow your environment to be visualized in the minds of your readers, so you have to be able to describe it in detail.
What JC said. Read in the genre to provide ideas on how to create and then introduce the world to the reader, allowing them to understand and move through it during the course of the story. Blend the methods with your writing style and story being told. There's no shortcut/three sentence answer on how to accomplish it. Plus, there is more than one way to accomplish the task.
While there is no easy way to create one, I know that one thing that really helped me was exactly what quincarroll suggested, drawing a map. Once I did that, not to mention I've read a few books similar to what I'm writing and that definitely helped, I was able to really see where things were and where I might need to place mountains, where I wanted forests, things like that. I think it's important to read a lot of books and also to draw maps. If you can see it in your mind, you can eventually put it on paper. It's a start at least.
