Yes, starting early is a good first step, but you have to be aware that it takes time and practice to get your writing to a level that is “ready for prime time.” Stephen King says you should consider your first 1,000,000 words as practice, and Malcolm Gladwell says it takes 10,000 hours working at something to become proficient. Those numbers are pretty spot on from my own experience and others. So work on developing your voice, read voraciously, dissect why authors do what they do and how, and you’ll greatly increase your chances.
Having a first book written by the time you finish high school is a great goal! But I’m sorry to say the chances of it being good enough to publish is pretty small. As I mentioned above, it takes time and practice to develop your voice and the tools needed to be successful. My first published novel was the 14th I wrote. And the first 10 were pretty horrific. But I got better with each one…that should be your immediate goal.
Most authors (even those who have a good amount of success) don’t earn a full-time living from their writing…especially early on). So it’s best to choose a position that will give you time to write. For instance, a night clerk at a hotel is a great choice as there is little to do and you can write when nothing else is going on. A night security job also has that type of “down time” that allows you to write while you are being paid to “stay awake” at the job you are being paid for. Teaching is another great choice because you have the summers off. What you don’t want is a high pressure job that is going to consume you 50 - 80 hours a week. Those professions won’t give you much free time for writing.
I hope some of this helps.