If you've ever told anybody, at any point in your life, that you want to be a writer then you've probably heard one very shopworn piece of advice: write what you know. I've decided that they might not be talking about plot points and subject matter, however. I'm pretty sure they're talking about the words.
Reaching and Overreaching
Everybody has used a word the wrong way. But when you're an author, you're not really supposed to use them incorrectly. Do it in a book, and some savvy reader is definitely going to catch on.
Don't use words you don't really know when you're writing. If you're reaching for words, you're just taking yourself out of the flow of writing the story. Use the words that come naturally when you're writing, and your writing will come to you a lot easier.
If you find that you're using too many of the same words over and over, you need to learn new words. Expand your vocabulary by reading more books. Look for books in your genre and books outside of it, because it never hurts to broaden your horizons. You'll pick up more words naturally, and that will improve your writing.
You don't need a whole lot of words to express a great story. Being overly descriptive will not serve your plot well, and it will only wear out your readers. Don't worry about using a lot of fancy words. Just use the ones that express exactly what you want to write.