Great question, and I do hope you pursue your passion of writing!
I hate to be the bearer of bad news, but I do think it's important to make decisions based on all the facts and information possible!
The average U.S. author makes below the poverty line each year: http://www.publishersweekly.com/pw/by-topic/industry-news/publisher-news/article/68008-new-guild-survey-reveals-majority-of-authors-earn-below-poverty-line.html
There are a whole host of reasons that combine to create this bad financial situation for authors, including:
Crowded Market: With over 300,000 new releases per year in the United States alone, the book market is positively flooded
No-limit Returns: While other products have perhaps thirty days of returnable time, bookstores can return books to the publisher whenever they please; in fact, as of 2008 roughly 25% of all books sold are returned. http://www.npr.org/templates/story/story.php?storyId=91461568
*Small advances and bad royalty rates: literary agent Rachelle Gardner estimates the average advance (money authors get up-front from publishers) is $5,000 to $15,000. This money is meant to cover the entire writing and editing process, AND most authors will never "earn out" their advance. Even if you do earn out the advance, royalty rates are often 8-10% for new authors.
If that was all too long, the summary is: most authors don't earn enough money to support themselves on books alone.
Does that mean you shouldn't be an author? Absolutely not! You can plan to write books AND supplement your writing earnings with other income sources. For instance, I know many authors who also do freelance writing and editing on the side, or teach English but write fiction at night and on weekends. There are a ton of career options that use writing as a main skill that aren't being an author: technical writer, content writer, copy editor, working in a publishing house, marketing, social media, etc.
You can pursue your dream of being an author AND plan for financial security. There are a rare few authors who do make a full-time salary from their books alone, but just in case you aren't one of those, you can have other sources of income.
I hope this helps, and please feel free to ask if you have any questions!
Sources: worked in publishing for two years, know many authors personally, research
Your income will vary. I it will depend on if you're self publishing or if you have a major publisher backing you. Self-publishing can be very rewarding now days. Technology has really changed and it allows authors free exposure across the world. I know several self published authors who rely on writing as their major income because of social media. Social media is where its at. Self-publishing also allows you to keep 100% of your book sells vs signing with a publisher where you will only get an estimate of 15%.