The most important rookie mistake is breaking your own rules. If you decide that magic in your world is an innate thing that certain people are born with the ability to use, then don't let a random character suddenly be able to throw fireballs. And do not create new abilities as the story grows without a good reason: for example, while a burgeoning wizard can easily learn new skills that a reader would not know about, the master in your story should not ignore a basic and useful spell just because you don't want the reader to know about it yet - once the reader finds out, they'll be confused as to why they didn't use it earlier. You're building a world; make sure that your rules are consistent, and that your characters act in accordance with these rules in a consistent manner. Failure to do so will make your writing seem amateurish.
Embrace tropes, but don't overuse them. Feel free to take on the hero's journey, as it's a very good plot outline, but make sure you provide your own twist: make sure that all the secondary characters get their own plot and are not just supplementing the main hero, for example, or cut off a plot prematurely by killing a character (and forcing the living characters to deal both with the death of their companion and the unresolved plot). Feel free to have a dark lord that the heroes need to defeat in order to stop evil from conquering the land - but make sure the "land" they are defending is full of flawed and less-than-good people as well, who may just use this rising threat as a way to further their own agendas. Tropes are good; embrace them and adapt them to your own story.
You decide what voice your story has. Feel free to have a modern conversational tone if you like, or go for a Tolkien-style novel with elevated language that feels "epic" and not of this world. Just stay consistent throughout. Once again, and I cannot stress this enough, inconsistency will take the reader out of the story.
Don't worry about "overused" words. That said, don't use an adverb when another word or phrase would do the trick. "He ran into the room quickly and immediately told us about the enemy's plans" gets the point across, but doesn't evoke the imagination as much as, "He burst into the room, panting as he told us the enemy's plans." Adverbs tend to "tell," where as phrases tend to "show." But don't avoid adverbs for the sake of avoiding adverbs.
Pick a "culture" to make up words - or to take words from directly, if you'd prefer. A lot of Celtic, Latin, and Norse words have been used in fantasy stories over the years, and other ancient languages such as Egyptian, Hebrew, and Greek are great places to borrow words from.
