Any tips on how to create really good terrains?

Use this 2 tutorials:

http://kostiantyn-dvornik.blogspot.com/2013/01/unity-4-cool-sweet-terrain-tutorial.html

http://kostiantyn-dvornik.blogspot.com/2013/11/awesome-unity-texturing-terrain-tutorial.html

With Fantasy work, In my opinion get yourself pleanty of models of your houses / castles/ villages, variation of trees and plants etc first. I find that once the assests are built and you have grand variation of tress, plants, bushes etc it all comes together and you can use your props to get an average estimate of what height is what in your world that way your mountains etc would be at the proportions you want relative to your props. (This stands for anything open world (I am using Elder Scrolls style as my example))

The above is probably what I think is best as your estimated proportions would be much better compared to starting of with the terrain and it becoming much smaller or larger than your character.

When it comes to singular maps with the terrain tool in Unity I find it is a quick solution but does not often give the best results. I find building maps/scenes in a 3D Program and using them in your scenes with a mesh collider is the best option as everything is proportional to one another and you can scale it to the size of your character or scale the character to the size of your scene. An example of this is the FPS Tutorial from Unity. Though this method is only Ideal when your working in maps and not a open world

This is a design question which is very much forum based and not Unity Answers which is mainly directed at programming issues, though I gave you my opinion on the matter.

Hope this helps somehow. Plan it out, get used to the feel of the terrain tool.

There is not a simple way to answer and haven't an unique answer, it's a very embracing subject.

But you can find some great articles in many websites like this
http://worldofleveldesign.com/

A hint: before start to make your world, choose carefuly a theme, seach for references images about your theme in the web, waste at least 2 days doing this, even boring this can be, start planing on a paper some rafts, and, after all this , when you are sure that your rafts are right, start to modeling and texturing.

i face the same problem,http://worldofleveldesign.com/ is good but does not focus on terrain.For ideas check out this page-

http://www.worldofleveldesign.com/categories/level_design_tutorials/100_level_design_ideas/100_level_design_ideas_and_locations.php

If u do find any other websites please post it