When looking for World-building resources, the obvious inspiration are worlds others have built. While there are a multitude of great fantasy worlds out there that are psuedo-medieval earth with some added magic, you´ll probably want a few that area a little bit different.
Let´s start with what I would consider two of the best worlds for using as inspiration in the fantasy genre.
A savage dark fantasy world (called Athas) built for Dungeons and Dragons 2nd Edition, it is dominated by a blasted desert created by the ravages of arcane magic. There are no gods here and psychic abilities are common. The setting had a more recent lease of life when it was re-released for D&D 4th Edition in 2010.
Most of the typical dungeons and dragons conventions were turned on their heads. The world is full of weird creatures, fantasy races far from their typical roots and unusual locations. The environment itself is one of the main protagonists of the settings, where just surviving from day to day is an achievement in itself.
Among the highlights are the wilds-dwelling cannibalistic halflings who dwell in the narrow Forest Ridge; the forever - desert-running nomadic elven tribes who focus on the present; the vast Sea of Silt which cannot support the weight of a person, becomes hard-packed a few metres down and is sometimes sailed by special ships
Next up is the wacky world created by Terry Pratchett, the famous English writer of comic fantasy. The Discworld has a unique slant on fantasy, taking many traditional tropes and casting them in a satirical and colourful manner.
Carried on the back of a giant turtle through space, the Discworld balances on the back of 4 elephants on said turtle (named Great A'Tuin). This sets a fair tone for the world.....showing what can be done when taking traditional fantasy tropes and twisting them just a little. It's full of unique and flavourful characters such as the Luggage, a case with many legs and feet that follows it's owner everywhere and defends itself with deadly teeth and tongue; Death, the practical escort of souls to the afterlife AND WHO SPEAKS ONLY IN CAPITALS; Cut My Own Throat Dibbler, a street merchant and entrepreneur of Ankh-Morpork selling meat pie products, snow globes, movies and intangibles;
The Witches, the Wizards and the Night Watch are all groups which feature over several books and are great inspiration for other organisations, featuring rich interplay between the diverse members. Ankh-Morpork is a prime example of city-building with a functioning example of Assassins' and Thieves' Guilds
The best ways to get to grips with the Discworld is probably by reading a few of the books. Among my recommendations would be Guards! Guards!, Small Gods, Reaper Man and Going Postal. There are two great wiki site at wikia.com and ispace.org. Or there is a dead tree guide in the New Discworld Companion, an updated version of the original Discworld Companion. The Discworld has also been featured in audio and television formats!