No matter what kind of fiction you write, you need do to research. In my case, a lot has been historical research for self-explanatory reasons. As a non-native English speaker, however, I have also had to investigate words and their various meanings, in fact more than any other topic. I would like to share the website with you, which has been my most important resource while writing my book. Whether you write fiction or something else, whether English is your first language or not, I think you might also find this site useful: www.etymonline.com
Think of it as an advanced dictionary, explaining not just words but their origin, current and former meanings, and many other useful facts. This is what we call etymology, the history of words, detailing each word and its place as a small twig on the great trees we call languages.
For a fantasy writer, etymology is especially important to get right. Let me give you an example. In the English-speaking world, John is a very popular name. It is an abbreviated form of the Medieval Latin Johannes, which added the H from Late Latin, which spelled it Joannes. Latin got the name from the Greek Ioannes, which got the name from the Hebrew Yohanan. Why is a Hebrew name so popular in the Germanic language of English? As you can probably guess, because of the Bible's influence upon English culture and subsequently English language. In case it is not obvious, all of the etymological information in this paragraph was taken from etymonline.com; it is a good example of how thorough the site is.
If you are writing a fantasy book set in a world independent of ours, that same development of the name John obviously cannot have happened. I would advise you against naming your character John, unless you have an in-universe explanation for how the abbreviated English form of a Hebrew name that went through the Greek and Latin wringer came to be. There are countless more examples like this. One of my personal favourites is the word Arsenal; try going to the site and look it up to see its journey from Arabic via Venice to becoming an English football club.
It is impossible to be certain that you never use a word, which etymologically clashes with your fantasy world. There are just too many words in the English language for any one writer to know the origin of them all. This site is a good place to start, however, no matter what you write or what you need to know in regards to the English language.