It depends on what kind of writing you want to do. For fiction, you could start by submitting writing to short story contests or literary magazines. For nonfiction, it depends on your interests. Buzzfeed and thoughtcatalog churn out a shitload of short-form articles written by young adults. If you have a facebook, you can “like” these pages and the algorithm will start suggesting similar articles. That will give you a good idea of the writing style and subject matter. (Mostly short articles on the trials and tribulations of 20-something life.)
You might also want to join a writing group or creative writing class. The feedback will help, and the other members or the instructor might have ideas about how to get published. Published writing should be clear, stylish, and efficient.
Lastly, some people self-publish in the kindle marketplace. My… ~background~ is in fandom, and there’s a lot of discussion going around that people can make money selling short-form stories. The most successful genres are romance and erotica; apparently people sell small, chapter-length stories for 99 cents to three dollars. Obviously that’s not for everyone, and you’re responsible for editing, marketing, cover design etc. yourself.