Whether you want them to or not, the words you choose say a great deal about you. Language is a powerful thing—more powerful than we probably realize most of the time. The words we use present an image of us to the world, just like the clothes we wear or the way we carry ourselves.
And as authors, we are not only responsible for our own words, but the words of our characters, too. A common piece of writing advice is to make sure everything in your book matters to the story: every scene and character interaction adds something important. None of it is there just to fill space.
This is true of words, too. It isn't about which words you use, but how you use them. I've seen people say, in argument to this, that it would be impossible to read through your manuscript and ask yourself whether each individual word moves your book forward. And, well, yes, it would. But that's not the point. The point is awareness of language.
Think swear words, for an easy example. They don't stand out if they're in keeping with your character, but if your character is the sort of person who would never, ever swear, and suddenly she drops the f-bomb, readers will take notice. You want them to take notice because you used the swear deliberately and it says something about the situation, or about the character. You don't want them to take notice because you slipped the swear in without thinking and it reads incredibly out of character.
Or uncommon words. There is a non-YA series of books that I love, but even when I read the first couple books in middle school, I noticed that the author used the word "acquiesce" with abnormal frequency. I started to notice it every time it was used, and get pulled out of the story. Overuse of uncommon words generally happens to me (and probably everyone?) when I'm drafting—in my last WIP, people's stomachs kept roiling with various emotions; in my current WIP, I am noticeably overusing the word viscous. But the hope is to cut them down in subsequent drafts so people aren't pulled out every few pages thinking, "hey there's that word again!" The same goes for using words in your characters' thoughts and dialogue that don't fit with the information you've provided us about them—someone who hasn't been to school and doesn't know how to read probably won't be using your fanciest SAT words, for example.
And here's where I maybe get a little soapboxy. Sometimes teenagers (and adults, for that matter) say things that are not good. That are hurtful to whole groups of people. For example, saying "that's retarded" as a casual insult. This is real-life dialogue. I've even heard people say it at work. But I think this is where it's important to be extra careful. If your character tells someone they're gay, who, in actuality, are they insulting? Are they insulting the other character, or are they insulting potential readers of your book? If your character judges other girls by calling them sluts, is that supposed to make us feel good about the character, or bad? To me, it's the latter, in both cases. If the characters you write say things like this and move on like it's no big deal, you're perpetuating the idea that it's okay to say these things casually, which isn't something that should be perpetuated. And it isn't that you should censor yourself, not at all. It's important to be aware of the message you're sending with the words you put on the page. To be aware of the feelings of the people who might one day read your novel. So if your character is going to call someone a whore, think about why that word and not another word. These are the sorts of words to pause over, to wonder if they're necessary or if you should think of something more fitting. And sometimes they really are necessary. I read a YA novel recently that did this really well--one character called another a retard, but he did it with specific intent, knowing exactly how badly it would make the other character feel. I cringed and was disappointed in the character who said it, but I know without a doubt that the author meant for me to feel that way. It was a character building moment, and the deliberateness of it was very clear.
And that's really what I'm saying with this post. Be deliberate. Use your words wisely, because words are powerful. And there's pretty much no way to say this without sounding like I'm pulling quotes from Spiderman here, but it really is true that power comes with responsibility. So use it well.