The way a writer uses language in their novel can either make it distinctive and fresh, or dull and uninspired. It's one of the weapons in your writer arsenal so make sure you know how to handle it like a pro.
Slang is one way to use language to your advantage. However, you have to be careful that it isn't something that will be obsolete or unpopular by the time your book comes out. Don't forget, finding an agent, then a publisher, and getting published can take years. The beauty of slang is that you can create your own, the catch is that you have to make sure your readers understand what it means. Anyone who loves sci-fi can tell you what frakkin means and from which series it originated. In my novel there are no clocks and time isn't thought of in minutes and hours. They use sundials and think of time in shadows of the sundial instead. I had to clarify that so the reader understood it.
Use your character's ethnic background to add flavor to the language of your novel. Throw in words from the language of their ancestors now and then in conversation when appropriate. If your character grew up speaking or hearing more than one language then there's a good chance that will come natural to them. Do your research and get it right though!
Be careful with offensive language, and I'm not just speaking from a young adult writers perspective. Sometimes by putting too much offensive language into your novel you run the risk of limiting your audience. Just like exclamation points, adverbs, and adjectives, cussing is more powerful when used in moderation.
Have fun with language, you're a writer, use it to your advantage!