When writing fantasy, not everyone chooses to go into details of their world. Then there are those (like me) who delve into it to create everything, literally, from the ground up.
So if you write fantasy and are trying to decide how deep into the creation process you want to go, there are some things you might want to consider first.
One thing to think about is your Pantheon. Gods and Goddesses, demi-gods, lesser demons and deities. Are you going to talk about them all the time? Will they be prominent in your writings or will they sort of be assumed and lurk in the background? If you want your characters to use their name in vain as in swearing, you might want to give some thought as to what they're cursing. If they're just going to be mentioned as almost an afterthought, then you might just want general descriptions. If they'll be more prominent, you may want to come up with short descriptions, what they represent and how they came into being.
Next is kingdoms, villages and cities. Figure out borders, even if they are only going to be in your own mind as you write. If you are like most writers and have far too much information in your head to begin with, draw out a map. Not an artist? Not a problem. Anyone can draw a few squiggly lines in a circle or oval shape and go from there. Some things to remember are main roads used in transporting goods through the Trade Route. Don't put down every single line that might end up just being a footpath. Determine where you want lakes, rivers, mountains and even swamps. Not exactly sure how to figure this out? Try looking at a map for an area around you. Perhaps even a map from another country. Most people who write fantasy try to base their maps off of familiar surroundings.
Decide whether your kingdoms will be Matriarchal or Patriarchal. Matriarchal societies are ruled by women while Patriarchal is ruled by men. Your societies can be both with a dominating Matriarch or Patriarch. Also decide if the heirs to the thrones will be the eldest male, eldest female or just the eldest regardless of gender. This will also help you determine your armies. Ranks within the military are listed if you google it or you can create your own ranks. For example, one of my kingdoms has a position of Guardian of the Realm. Basically it is a fancy title for the Captain of the Guard, yet is a step above that rank and orders filter down from the king to the Guardian, then on to the Captain and so forth.
Money. Money rules all worlds, imagined or otherwise. However, if you don't want to get into monetary values, just go with a general rule of gold, silver and copper coins. You can also have a society based on bartering. That way everyone would be somewhat equal, rich wouldn't be a description as there is no money.
Creatures are just as important to imagined worlds as they are to real ones. Animals and plants play a large part in our ecology, so think about what you would like. Staying to the mainstream foliage and animals is fine. Don't feel that you have to come up with a Jackolope or something to that effect if that is not your strength. If you do want to include some strange and interesting creatures or plant-life, make sure you have good descriptions.
World creation can become tedious unless you are basing your writing works off of your world. Personally, I spent almost ten years creating my world so that I would know it almost as well as the real world I live in. It makes writing easier for me and I know every creature, god, demon and king. I know where every road leads, I know the secrets of the earth that have remained undiscovered and how the mythical beings were created. It can be a daunting task sometimes and I do have to take a break from it now and then, yet I am pleased that I put so much time into the creation for lots of different reasons. Everyone is different, though and world creation is not for everyone who writes fantasy. To each his own. Happy building!