You’ll have to spend months, maybe years, world-building in fantasy, so be sure you’re prepared for the amount of time this will covet from your life.
For fantasy writers, this tends to be the ultimate challenge- concocting a world entirely from scratch.
Before starting, have a story in mind. Is it your intention to write a story where your protagonist(s) will be taking a great journey? Then you have to know every single detail of your world. Many authors tend to reflect their worlds on an old version of our own, which isn’t necessarily a bad thing, but I’ve heard from readers that sometimes they like something a bit more innovative. How to do this, you wonder? Do your research about every single thing in our world (our politics, customs, etc), then research times long past (if your world has a medieval setting, look at medieval times. If it has a prehistoric setting, research prehistoric times) and go on from there. Then change whatever is necessary to suit your world. If it was custom to greet each other by bowing, juggle with this- maybe merge two existing greeting versions into a new greeting version.
Something that really helps with world-building: Movies. Fantasy movies, specifically. Why? Because of their setting. Look at what has been used to create a believable setting and how it applies to our real world. No one place is like the other. We all have different religions, customs… and within one same country, each location is different. To give you a few easy examples to build on: how do you distinguish a village from a city in your novel? How do you distinguish a country in the north from one in the south (and no, an icy territory in one and a sunny territory in the other won’t cut it. You have to delve much deeper)? Movies are handy because if you can imagine your world in a movie-like setting, then you’re on the right path- that’s half of the problem solved. The next half of the problem, of course, is writing it in a way that your readers see it also.
The language… oh, the language! Please stay away from inventing languages if you’re not a linguist like Tolkien was or if you’re not planning to thoroughly research into the basic requirements for the production of a language. Every time I read blabber in a novel, I physically wince. Don’t make up non-existent languages with no true basis. What you can do, however, is that aside from their own “national” language, all beings in your world could speak a universal language. In this case, when they do speak in their own language, you can describe the sounds they make or how your protagonists can’t make out a single word. This is more believable because it happens to us too. When a person isn’t familiar with a language, it’s entirely possible not to even catch a word. Think about when you hear Chinese or Arabic, unless you’ve studied these languages or you were born there, you won’t know what’s being said. At times, you might be able to derive some kind of tone. Is the tone angry, glad, neutral etc? Describing tone is always a good thing because it drives us to employ our hearing sense. The more you can relate to the senses, the more people can relate to the situation themselves. If one person in your novel does speak a foreign language, you can always say “he said in insert-language” since this, again, avoids nonsense blabber that just takes up space.
One important thing: Do not plagiarize Tolkien. While he is one of the greatest fantasy world-builders, bear in mind that learning does not equal stealing. Learning is coming up with a way to use what you’ve been taught and create something new. It’s not ripping off Tolkien and reproducing a same-genre-different-version kind of novel. Also be mindful that the stereotypical beasts- Elves, (another version of) Orcs, Dwarves and so on- might be considered clichés by some and that said readers may want a writer to be more original.
Good luck with your world-building!